Log Message:
Update to 2.24.0. pkgsrc changes:

- Modernize patch filenames
- Take MAINTAINER

From the upstream changelog:

2.24.0:

          Note: LilyPond releases can contain syntax changes, which may
          require modifications in your existing files written for older
          versions so that they work in the new version.  To upgrade
          files, it is *strongly recommended* to use the 'convert-ly'
          tool distributed with LilyPond, which is described in *note
          (lilypond-usage)Updating files with convert-ly::.
          'convert-ly' can perform almost all syntax updates
          automatically.  Frescobaldi users can run 'convert-ly'
          directly from Frescobaldi using "Tools > Update with
          convert-ly...".  Other editing environments with LilyPond
          support may provide a way to run 'convert-ly' graphically.

Major changes in LilyPond
*************************

   . LilyPond now requires Guile 2.2.  Even if you are not writing
     Scheme code yourself, you may be using libraries that contain a
     non-trivial amount of customizations.  If they do not work with
     LilyPond version 2.24.0, please report this to the library's
     developers.  If you _are_ a library developer, *note Notes on Guile
     2.2:: below.

   . The infrastructure for creating the official binaries has been
     completely rewritten, incident with the switch to Guile 2.2.  As of
     this release, we provide 64-bit binaries for macOS and Windows.
     Also, all packages are made available as simple archives that can
     be extracted to any "installation" location.  To uninstall, simply
     delete that directory.  We also discontinued the limited editor
     that was installed on macOS and Windows, LilyPad, and recommend
     switching to an external solution instead, such as the popular
     editor Frescobaldi (https://frescobaldi.org), or one of the others
     listed in *note (lilypond-web)Easier editing::.  For more
     information, please refer to the detailed instructions in *note
     (lilypond-learning)Installing::.

     The version of Ghostscript that ships in the official binaries no
     longer includes its own fonts, to reduce download size.  LilyPond
     does not need them (it has its own set of default fonts).  This
     does mean, however, that code such as '/Arial findfont' in '\markup
     \postscript' no longer works.  We recommend using normal '\markup'
     syntax to print glyphs.

Known issues in LilyPond 2.24.0
===============================

On Windows, compiling very large scores (several hundreds of pages) can
result in crashes.(1)  We hope to address this in a future 2.24.X
release.

   ---------- Footnotes ----------

   (1) Note that very large scores could already crash in LilyPond 2.22
due to restrictions imposed by the 32-bit architecture.

Notes for source compilation and packagers
==========================================

This section is aimed at enthusiasts compiling LilyPond from source and
packagers preparing LilyPond for distribution.  If you are not part of
either group, you can skip over this section.

   . As mentioned above, LilyPond now requires Guile 2.2.  If needed for
     distribution reasons, it can also be compiled with Guile 3.0 by
     passing 'GUILE_FLAVOR=guile-3.0' to the 'configure' script.
     However, this is not yet recommended nor officially supported.

   . The Scheme code evaluator in Guile 2.2 is slower than in Guile 1.X.
     To offset most of the performance penalty, we recommend compiling
     the '.scm' files into bytecode by first running 'make bytecode'
     during compilation and then 'make install-bytecode' in addition to
     'make install'.

   . Starting with this stable release, LilyPond's build system does not
     install text fonts anymore.  Please provide them as separate
     packages while paying attention to the fonts' license and notice
     files.

New for musical notation
************************

Pitches improvements
====================

   . Support for alternate accidentals was improved.  Through the
     'alterationGlyphs' property of staff-like contexts, accidental
     glyphs may be set for all grobs at once (refer to *note
     (lilypond-notation)Alternate accidental glyphs::).

   . Ottava brackets may apply to a single voice instead of the entire
     staff.  This used to require workarounds.

Rhythm improvements
===================

   . The new '\section' command inserts a double bar line that interacts
     gracefully with repeat bar lines.  A passage can be named with the
     new '\sectionLabel' command.

   . '\numericTimeSignature' and '\defaultTimeSignature' now act on all
     staves at once (more precisely, on all staves in the same 'Timing'
     context), thus matching the behavior of '\time'.

   . The '\rhythm' markup command has been added.  It is a simple way to
     enter rhythms mixed with text, such as in "swing" indications.

   . The '\enablePolymeter' command is now provided as an input
     shorthand for moving engravers as is necessary to allow different
     time signatures in parallel.

     Independent of this, 'Default_bar_line_engraver' has been removed.

   . The new option 'visible-over-note-heads' can be used to make tuplet
     brackets always appear when their direction is set to be over the
     note heads.  It can be used with the default tuplet bracket
     visibility style or with '#'if-no-beam'.

   . Measure counts now take compressed multi-measure rests and
     alternatives into account.

   . Bar numbers may be centered in their measure, as is common in film
     scores.

   . The alignment of bar numbers appearing in the middle or end of a
     system has been changed to align them on their left edge.  This is
     in keeping with the advice of Elaine Gould ('Behind Bars', p. 237),
     and was mostly the consensus reached in a discussion of the issue
     by developers.  The alignment of bars at the beginning of a system
     remains unchanged.

   . '\bar ","' creates a short bar line.

   . The following predefined bar types no longer appear as a single bar
     line at the end of a line.  Annotated bar types (e.g., '\bar
     "S-|"') have been added for that purpose.

   . '\bar ""' is no longer necessary to print the first bar number.  It
     now suffices to set 'barNumberVisibility' to
     'all-bar-numbers-visible', or one of the other visibility settings
     where the first bar number is visible.

     Note that this is a change in behavior for scores that set
     'barNumberVisibility' to 'all-bar-numbers-visible' or such and
     'BarNumber.break-visibility' to '#t' without having '\bar ""'.
     Now, a bar number is printed at the beginning.  This is just the
     expected behavior (_all_ bar numbers should be visible), but due to
     slightly unclear documentation, users may have used these settings
     to print bar numbers in the middle of systems except for the first
     bar number.  In such cases, simply remove '\set
     Score.barNumberVisibility = #all-bar-numbers-visible' since
     '\override BarNumber.break-visibility = ##t' does the relevant
     setting alone.

   . The '\break' command now always inserts a break, bypassing all
     default decisions about break points.  For example, it is no longer
     necessary to insert '\bar ""' to obtain a mid-measure break.

     The new '\allowBreak' command inserts a possible break point,
     without forcing it, but bypassing default decisions like '\break'
     does.

   . The bar line type '"-"' has been removed.  'convert-ly' converts it
     to '""'.  There is a slight difference in horizontal spacing at
     line breaks.

   . 'automaticBars' has been removed.  'convert-ly' converts
     'automaticBars = ##f' to 'measureBarType = #'()'.

   . '\defineBarLine' now accepts '#t' in lieu of repeating the mid-line
     glyph name.

   . 'Bar_engraver' used to forbid line breaks between bar lines in all
     cases, but now it only does so when the
     'forbidBreakBetweenBarLines' context property is set to '#t', which
     is the default.  The 'barAlways' context property, which previously
     worked around the lack of 'forbidBreakBetweenBarLines', has been
     removed.

   . Due to changes in the internals of '\bar', it is no longer
     supported to use it before creating lower contexts with '\new'.
     Such uses will now create an extra staff.  This is similar to what
     happens with commands such as '\override Staff...' (see *note
     (lilypond-usage)An extra staff appears::).

     The solution is to place '\bar' inside the music for each staff, as
     is usual with most commands.

   . The bar type '"-span|"' creates a mensurstrich.

   . 'Staff' contexts use the new 'Caesura_engraver' to notate the
     '\caesura' command.

Expressive mark improvements
============================

   . Events attached to notes (e.g., dynamics or articulations) can be
     delayed by an arbitrary duration using '\after'.  This simplifies
     many situations that previously required the use of explicit
     polyphony and spacer rests.

   . Broken hairpins now have some left padding by default.  This is in
     line with published scores and it fixes some cases where broken
     hairpins were vertically displaced by the key signature.

   . The ends of hairpins may now be aligned to the 'LEFT', 'CENTER' or
     'RIGHT' of 'NoteColumn' grobs by overriding the property
     'endpoint-alignments'.

   . The direction of a trill spanner can now be set with direction
     indicators like other articulations, i.e.  with '_\startTrillSpan'
     or '^\startTrillSpan'.

   . The default appearance of trill spanners has changed to better
     match classical engraving conventions.  They now end before the
     next note, not over it.  If the next note has an accidental, they
     stop before it.  If the next note is the first note of a measure,
     they stop over the bar line instead.

   . The padding by default for fermatas is now larger.  This avoids
     some cases where the fermata was placed too close to dots and other
     objects.

   . The flageolet symbol is now smaller and slightly thicker.  This is
     in line with published scores and makes the recommended workaround
     to make it smaller ('\tweak font-size -3 \flageolet') unnecessary.

   . The accent glyph is now a bit smaller.  This fixes some cases where
     a natural sign would vertically displace accents.

   . The comma glyph shape, as used in the '\breathe' command, has been
     changed to a more common form.

     The old glyph remains available under the name 'raltcomma'.

   . The new context property 'breathMarkType' selects the mark that
     '\breathe' produces from several predefined types.

Repeat improvements
===================

   . Repeat alternatives may appear within the repeated section.

   . The volta numbers for repeat alternatives may be set with the
     '\volta' command.

   . The new '\repeat segno' command automatically notates a variety of
     _da-capo_ and _dal-segno_ forms.

   . The new '\fine' command inserts a final bar line that interacts
     gracefully with repeat bar lines.  Used inside '\repeat', it also
     prints _Fine_ and ends the music after unfolding.

   . The '\volta' command removes music when a repeat is unfolded.

   . The '\unfolded' command adds music when a repeat is unfolded.

Editorial annotation improvements
=================================

   . The new '\staffHighlight' and '\stopStaffHighlight' commands can be
     used to color a musical passage.

   . A new grob 'FingerGlideSpanner' is now available, indicating a
     finger gliding on a string from one to another position.  Several
     appearances are possible, depending on the setting of 'style'.
     Shown in the image are 'line', 'stub-left', 'stub-right' and
     'stub-both'.

     Also possible is 'dashed-line', 'dotted-line', 'zigzag', 'trill',
     'bow' and 'none'.

   . Balloons now have changeable formatting.

   . Parenthesizing chords is supported.  Currently, the font size of
     the parentheses has to be adjusted manually.

   . Parenthesizing spanners is supported.

   . A "time-based" version of the '\parenthesize' command was added.
     It takes a grob path: '\parenthesize GROBNAME' or '\parenthesize
     CONTEXTNAME.GROBNAME'.  It acts like a '\once \override'.  This
     interface complements the already existing form '\parenthesize
     EVENT', in a fashion similar to '\footnote'.

   . Adding the 'Melody_engraver' to the 'Voice' context now works out
     of the box to change the stem direction of the middle note
     according to the melody.  Previously, this required an additional
     override to 'Stem.neutral-direction'.

     The 'suspendMelodyDecisions' context property may be used to turn
     off this behavior temporarily, as '\override Stem.neutral-direction
     = #DOWN' used to do.

   . The new 'Mark_tracking_translator' takes over from 'Mark_engraver'
     the decision of when to create a mark.  'Mark_engraver' continues
     to control formatting and vertical placement.

     By default, 'Mark_engravers' in multiple contexts create a common
     sequence of marks.  If independent sequences are desired, multiple
     'Mark_tracking_translators' must be used.

Text improvements
=================

   . New commands '\textMark' and '\textEndMark' are available to add an
     arbitrary piece of text between notes, called a text mark.  These
     commands improve over the previously existing syntax with the
     '\mark' command called as '\mark MARKUP' (i.e., '\mark "..."' or
     '\mark \markup ...').

     '\textMark' and '\textEndMark' are now the recommended way to
     create textual marks.  The use of '\mark' for this purpose is still
     supported, but discouraged (note that the '\mark' command itself is
     not discouraged, only calling it on a markup argument; '\mark
     \default' or '\mark NUMBER' is still the recommended and only way
     to create a rehearsal mark).

     The new commands have several differences to '\mark MARKUP'.  There
     can be an arbitrary number of them at a given moment, while there
     can only be one use of '\mark'.  They output grobs of the dedicated
     'TextMark' type, whereas '\mark' creates a 'RehearsalMark' grob
     regardless of whether it is called for a rehearsal mark or a
     textual mark; introducing this distinction allows stylesheets to
     set different layout settings for rehearsal marks and text marks.
     The alignment set by the new commands is different: '\textMark'
     always creates a left-aligned mark, and '\textEndMark' creates a
     right-aligned mark; in contrast, the alignment of a 'RehearsalMark'
     depends on the anchor point of the object it aligns to.

     See *note (lilypond-notation)Text marks:: for full details.

   . Text variant glyphs for sharp, flat, natural, double sharp, and
     double flat are now available in the Emmentaler fonts.  In markup,
     they can be easily accessed with standard Unicode values.

   . It is now possible to control the width and the shape of (some)
     Emmentaler digits using OpenType features.

   . '\smallCaps' now works on any markup, not just on a bare string.

   . The syntax for conditions in markups was made more flexible and
     user-friendly.  It uses the new markup commands '\if' and
     '\unless'.  Here are example replacements:

     2.22 syntax                              2.24 syntax
     --------------------------------------------------------------------------------
     '\on-the-fly #first-page ...'            '\if \on-first-page ...'
     '\on-the-fly #not-part-first-page ...'   '\unless \on-first-page-of-part ...'
     '\on-the-fly #(on-page N) ...'           '\if \on-page #N ...'

   . With the new markup list command 'string-lines' it is now possible
     to split a string at a given character.  The default is to split at
     line break.  Surrounding white space gets dropped.  The resulting
     list of markups may be further formatted.  This is a very
     convenient way of entering additional stanzas for songs.

   . The new markup command '\align-on-other' translates a markup as if
     it was aligned to another markup.

   . Two new markup functions '\with-dimension' and
     '\with-dimension-from' are available.  They are similar to
     '\with-dimensions' and '\with-dimensions-from', respectively,
     modifying only a single dimension (instead of both).

   . New markup commands '\with-true-dimension' and
     '\with-true-dimensions' are available.  They give the markup the
     actual extent(s) of its printed ink, which may differ from the
     default extents for some font glyphs due to text regularity
     constraints.

   . Text replacements can now replace strings with any markup, not just
     with a string.

   . A new markup command '\with-string-transformer' is available.  It
     interprets a markup with a "string transformer" installed; the
     transformer is called when the interpretation of the markup
     requires interpreting a string, and allows to perform modifications
     on this string, such as changing case.

   . The 'markup->string' function converts a markup into an approximate
     string representation; it is used for outputting PDF metadata as
     well as MIDI lyrics and markers.  Markup commands can now define a
     custom method to convert markups created using them into strings,
     for use by 'markup->string'.  For example:

          #(define-markup-command (upcase layout props arg) (string?)
             #:as-string (string-upcase arg)
             (interpret-markup layout props (string-upcase arg)))

New for specialist notation
***************************

Fretted string instrument improvements
======================================

   . The string tunings 'banjo-double-c' and 'banjo-double-d' were
     added.

   . A new grob 'BendSpanner' is now available for 'TabStaff',
     indicating a bent string.  Apart from the default three styles are
     possible: ''hold', ''pre-bend' and ''pre-bend-hold'.

Percussion improvements
=======================

   . The drum notation style 'weinberg-drums-style' was added.  It is
     based on Norman Weinberg's standardization work.

Wind instrument improvements
============================

   . Additional display details of a '\woodwind-diagram' can now be
     specified including the angle of partially-covered-keys and the
     display of non-graphical trill keys.

Chord notation improvements
===========================

   . Support for chord grids has been added.

   . In 'ChordNames', multi-measure rests now also cause the "N.C."
     symbol to be printed, just like normal rests.

   . In figured bass, '_' now creates an empty figure that still takes
     up space.

   . Formatting of figured bass has been improved.  In particular, the
     default size is reduced to a value used by many Urtext editions of
     Baroque music.

   . In figured bass, specially designed glyphs for '6\\', '7\\', and
     '9\\' are now used by default.  Similarly, specially designed
     glyphs for symbols '2\+', '4\+', and '5\+' are used by default if
     plus signs appear after the number.

     Use the new command '\figured-bass' to access these glyphs in
     markup.

   . In figured bass, brackets can now also be added around accidentals.

Ancient notation improvements
=============================

   . A new context 'VaticanaLyrics' is available.  It is similar to
     'Lyrics', providing a hyphenation style (a single, flush-left
     hyphen between two syllables) as used in the notational style of
     Editio Vaticana.

   . The predefined commands for Gregorian divisiones are no longer
     variations on '\breathe'.  '\divisioMinima', '\divisioMaior',
     '\divisioMaxima', and '\virgula' are variations on the basic
     '\caesura'.  '\finalis' is equivalent to '\section'.

     'MensuralStaff' and 'VaticanaStaff' use 'Divisio_engraver' to
     interpret the above commands as well as '\repeat volta' and
     '\fine'.

   . 'KievanStaff', 'MensuralStaff', 'PetrucciStaff', and
     'VaticanaStaff' now allow line breaks anywhere, and they no longer
     create '""' measure bar lines.

   . In 'GregorianTranscriptionStaff', divisiones are now engraved as
     'BarLine' grobs by default.  To change them to 'Divisio' grobs, use
     '\EnableGregorianDivisiones'.

   . 'GregorianTranscriptionStaff' allows a line break after any note
     and no longer uses 'Time_signature_engraver'.

   . 'GregorianTranscriptionVoice' no longer uses 'Stem_engraver'.

World music improvements
========================

   . Support for Persian classical music is now available.  For this,
     two accidental glyphs, _sori_ and _koron_, have been added to
     LilyPond.

Miscellaneous improvements
**************************

   . In the Emmentaler font, identical-looking noteheads whose only
     difference was stem direction have been consolidated into a single
     glyph.  For instance, the glyphs 'noteheads.u2triangle' and
     'noteheads.d2triangle' have been replaced by a single glyph,
     'noteheads.s2triangle'.  Notehead pairs that look visually
     different depending on the direction remain distinct.

     In addition, the 'stem-attachment' property of 'NoteHead' grobs now
     returns its actual, direction-dependent stem attachment point
     instead of a hypothetical upwards-stem attachment point.

   . Two redundant glyphs in the Emmentaler font have been removed:
     'scripts.trillelement' (use 'scripts.trill_element' instead) and
     'scripts.augmentum' (use 'dots.dotvaticana' instead).

   . Using '\paper { bookpart-level-page-numbering = ##t}', it is now
     possible to make bookparts independent with respect to page
     numbering.  If this is used for all bookparts, each bookpart has
     its own numbering sequence, starting at 1 by default.  It can also
     be used in an individual bookpart, which is useful to achieve the
     standard practice of numbers pages in an analytical introduction
     independently and in roman numerals (the latter is achieved using
     'page-number-type = #'roman-lower').

   . A new grob callback function 'break-alignment-list' is now
     available for returning different values depending on a grob's
     break direction.  As an example, use it to provide different
     alignments of a grob depending on whether it is positioned at the
     beginning, the middle, or the end of a line.

   . The new 'Mark_performer' creates MIDI Marker events like
     'Mark_engraver' creates printed marks.

   . Properties of 'PaperColumn' and 'NonMusicalPaperColumn' (such as
     'NonMusicalPaperColumn.line-break-system-details') can now be
     overridden mid-music with the usual command '\once \override'.
     They used to be a special case requiring the '\overrideProperty'
     command.

   . The new 'show-horizontal-skylines' and 'show-vertical-skylines'
     properties allow to display an object's skylines.  This is more
     flexible than the already existing 'debug-skylines' option because
     it works for all grobs.  While primarily meant for debugging
     LilyPond, this can be useful when trying to understand spacing
     decisions or overriding stencils in Scheme.

   . The new command '\vshape' is like '\shape', but also shows the
     control points and polygon for easier tweaking.

   . '\markup \path' now also works in SVG output even if the path does
     not begin with a 'moveto' or 'rmoveto' command.  Also, it now
     accepts single-letter SVG equivalents ('moveto' = 'M', etc.).

   . 'set-default-paper-size' and 'set-paper-size' now accept a custom
     paper size.

   . 'lilypond-book' supports two new music fragment options
     'paper-width' and 'paper-height' to set a custom paper size.

   . 'lilypond-book' supports a new snippet option 'inline' for inline
     music, that is, music snippets like [[image of music]] that appear within a \ 
paragraph
     of text.

   . The 'lilypond-book' script now allows braces in the argument of the
     commands '\lilypond' (for LaTeX) and '@lilypond' (for Texinfo).

   . 'lilypond-book' now appends the current directory as the last entry
     to search for included files, instead of prepending it to the list
     of specified include paths.  This allows include directories to
     shadow files from the current directory, and will only be noticed
     if there are files with the same name in both.

   . The new Scheme function 'universal-color' provides an eight-element
     color palette designed to be unambiguous to people with
     dichromatism.

   . The '-dembed-source-code' option now also embeds images added with
     '\epsfile' and files included with '\verbatim-file'.

   . The default of the 'aux-files' program option changed to '#f'.  If
     you are calling LilyPond with the '-dbackend=eps' argument and need
     the auxiliary '.tex' and '.texi' files, you now have to specify
     '-daux-files' explicitly.  The formats for 'lilypond-book' images
     can be set separately for the tall page image (typically PNG for
     HTML output) and per-system images (typically, EPS or PDF for
     printed output) with the '-d' sub-options '-dtall-page-formats' and
     '-dseparate-page-formats' respectively.

   . The 'big point' unit (1bp = 1/72in) is now available by appending
     '\bp' to length values.

   . Scheme-defined translators usable in both '\layout' and '\midi' can
     now be created with 'make-translator'.  Scheme-defined performers
     usable only in '\midi' can now be created with 'make-performer'.
     Those macros work strictly like the previously existing macro
     'make-engraver' for creating engravers only usable in '\layout'.

   . Scheme translators can now define a new slot called
     'pre-process-music'.  It is called on all translators, after all
     listeners but before all 'process-music' slots.  This can be used
     for processing that depends on all events heard but needs to set
     context properties before other translators read them.

   . Scheme translators can now contain listeners written as

          (listeners
           ((event-class engraver event #:once)
            ...))

     These are never triggered more than once per time step.  They emit
     a warning if they receive two events in the same time step, except
     if the events are equal.

   . The same grob definition can now be used to create grobs of
     different classes ('Item', 'Spanner', 'Paper_column', 'System').
     As part of this change, the grob types 'FootnoteItem' and
     'FootnoteSpanner' were consolidated into a single type 'Footnote'.
     Similarly, 'BalloonTextSpanner' and 'BalloonTextItem' are unified
     into 'BalloonText'.

     When the grob definition does not mandate a class, engravers should
     choose what class to create a grob with.  For authors of Scheme
     engravers, this means using either 'ly:engraver-make-item' or
     'ly:engraver-make-spanner'.  The utility function
     'ly:engraver-make-sticky' is provided to support the frequent case
     of _sticky_ grobs, such as footnotes and balloons.  It creates a
     grob with the same class as another grob and administrates parents
     and bounds.

   . The new command-line option '-dcompile-scheme-code', also settable
     in the LilyPond input with '#(ly:set-option 'compile-scheme-code)',
     provides with better diagnostics when running Scheme code leads to
     an error.  Internally, this uses the byte-compiler provided by
     Guile, instead of the interpreter.

     However, due to a limitation in Guile, this currently has the
     disadvantage of making it impossible to run more than a few
     thousand Scheme expressions.  Also, be aware that the Guile
     compiler has a few differences to the interpreter.  For example,
     constant parts of quasiquotes are made actual constants more
     aggressively, making code such as '(let ((x 4)) (sort! `(,x 3 2
     1)))' produce an error because the "cdr" of the quasiquoted list is
     constant, and it is an error in Scheme to mutate literal data.  (In
     this specific case, the code could avoid the issue by using the
     non-destructive 'sort', or by creating a fresh list each time with
     '(list x 3 2 1)'.)

     Furthermore, this option does not currently work on Windows.

Notes on Guile 2.2
******************

This version of LilyPond switches from Guile 1.8 to Guile 2.2.  This
section lists some of the most common incompatibilities that you could
have to deal with in order to upgrade your Scheme code.

   A full, detailed log of changes in Guile can be found in the 'NEWS'
file (https://git.savannah.gnu.org/cgit/guile.git/tree/NEWS) of the
Guile source.

   . The 'format' function now requires a boolean or port as the first
     argument.  This argument was optional in Guile 1.8.  In order to
     make the function return the formatted output as a string, like
     'format' does without this argument in Guile 1.8, pass '#f' for
     this argument, i.e., '(format #f "STRING" ARGUMENTS ...)' instead
     of '(format "STRING" ARGUMENTS ...)'.

   . The rules for internal (i.e., non-toplevel) definitions have become
     stricter.  Definitions are no longer allowed in various expression
     contexts.  This is no longer valid, for example:

          (if (not (defined? 'variable))
              (define variable 'value))

     The solution in this particular example is:

          (define variable
            (if (not (defined? 'variable))
                'value
                variable))

   . Strings now support Unicode characters.  Previously, a Unicode
     character was represented by several characters, and various
     functions were not tailored for Unicode support.

   . Some numeric functions now return exact results in more cases.  For
     instance, '(sqrt 4)' returns '2.0' in Guile 1.8, but '2' (an
     integer) in Guile 2.2.

2.22.2:

New for musical notation
------------------------

Displaying pitch improvements
.............................

   . An accidental glyph corresponding to SMuFL code U+E2E3 has been
     added (Extended Helmholtz-Ellis accidentals: Raise by one undecimal
     quartertone).

   . Quarter-tone note names are now available in all the input
     languages.

   . Setting 'suggestAccidentals' to ''cautionary' turns only cautionary
     accidentals into 'AccidentalSuggestion's.  This can be used to
     distinguish between facsimile and editorial accidentals.

   . The command '\ambitusAfter' has been added.  It is used to move the
     ambitus to a different position.

   . Ottava brackets are now introduced with a single number by default
     (e.g., '8' or '15'), and printed in bold.  A new
     'ottavationMarkups' property has been introduced to modify that
     behavior, with several predefined lists of markups available.

Rhythm improvements
...................

   . Tuplets can now be printed with slurs instead of brackets.

   . French beaming ('\override Stem.french-beaming = ##t') now
     _exactly_ behaves like standard (i.e., default) beaming in every
     respect (beam positioning and placement of any articulation,
     fingering, etc.).  The only remaining difference are inner stems
     not passing through beams.

   . Swing and irregular rhythmic patterns may now be applied to music
     expressions made of regular durations, which may be used to render
     inequal rhythmic interpretation in MIDI.

   . Font glyphs for 256th, 512th, and 1024th flags and rests have been
     added.

   . The new 'Merge_mmrest_numbers_engraver' hides duplicate numbers on
     multi-measure rests.

Expressive mark improvements
............................

   . A dynamic command '\n' for _niente_ has been added.

   . Two new ornaments have been added.

   . A very short fermata and Henze variants of long and short fermatas
     have been added.

   . Fermatas and other articulations can now be added directly to
     multi-measure rests.  Therefore the command '\fermataMarkup' is
     deprecated.

Editorial annotation improvements
.................................

   . The 'New_fingering_engraver' will now consider the
     'X-align-on-main-noteheads' property of the
     'self-alignment-interface'.  If set to true ('##t'), all fingerings
     oriented 'up' or 'down' will be arranged in a straight column and
     aligned on the noteheads on the correct side of the stem.

Text formatting improvements
............................

   . The default fonts are now those from the URW Core 35 set, version
     2.0, replacing the TeX Gyre fonts.  Note that the fonts come with a
     ligature for 'Nr.'; see *note (lilypond-notation)Fonts explained::
     for solutions to avoid it locally and globally.

   . The '\note' markup command now takes as its first argument a
     duration instead of a string.

New for specialist notation
---------------------------

Vocal music improvements
........................

   . A lyric hyphen may now be repeated at the start of a system
     beginning with a new syllable.

   . A gradual change of vowel (or sustained consonant) may be indicated
     by adding a vowel transition between lyric syllables with the
     command '\vowelTransition'.

Unfretted and fretted string instrument improvements
....................................................

   . Fret-diagrams may now be printed left-handed, setting 'handedness'
     to 'LEFT' (as a subproperty of 'fret-diagram-details').

   . Some ukulele fretboard-diagrams have been fixed, and new diagrams
     have also been added.

Chord notation improvements
...........................

   . Chords may now be automatically inverted or voiced with dropped
     notes.

   . Legacy chord naming functions 'banter-chord-names' and
     'jazz-chord-names', have been removed from the main codebase, as
     have been the properties 'chordNamesExceptionsFull' and
     'chordNamesExceptionsPartial'.  As part of that rewrite, power
     chords are now included in the default exceptions; they will be
     printed correctly (with a superscript like all other chords) with
     no additional tweaks required, thereby removing the need for the
     '\powerChords' predefined command.  Anyone interested in the legacy
     functions may find a (somewhat working) copy of them in the
     'chord-names-alternative.ly' snippet.

Contemporary music improvements
...............................

   . A new grob 'DurationLine' is now available.  It continues a
     rhythmic grob with a line, ending at the next rhythmic grob.
     Possible styles are ''beam', ''line', ''dashed-line',
     ''dotted-line', ''zigzag', ''trill' and ''none'.  The duration line
     may end with a hook (beam-style only) or an arrow.

New for input and output
------------------------

Input file improvements
.......................

   . As announced in version 2.17.3 nearly eight years ago, the
     ''relative-includes' option is now enabled by default; included
     files that contain an '\include' command of their own must account
     for their own path rather than the main file's directory.  That
     behavior may however be switched off by setting
     ''relative-includes' to '#f', either as a command line option or
     using 'ly:set-option' in source files.

   . '\compressFullBarRests' has been renamed to
     '\compressEmptyMeasures', to avoid possible confusion with
     '\compressMMRests'.  Likewise, '\expandFullBarRests' has now become
     '\expandEmptyMeasures'.

   . The '\partcombine' command, as well as all 'partCombine'-prefixed
     commands, subroutines and property names, are now written with a
     capital C, such as '\partCombine', '\partCombineApart' etc.  The
     '\autochange' command is now also capitalized as '\autoChange'.

   . All input languages ('\language' statement) can be entered using
     their proper UTF-8 spelling (i.e., including special characters).

Output improvements
...................

   . Skylines will now take account of the 'rotation' property of layout
     objects.  Rotating a crescendo hairpin by applying '\override
     Hairpin.rotation = #'(15 0 0)', for instance, will now actually
     have influence on the skylines and thus help to enhance spacing.

   . Slight padding added between natural glyphs just touching at the
     corners in key cancellations.

   . Skylines of boxes now reflect the actual box outline including
     rounded corners and rotation.

   . CSS-style colors can now be used directly as text strings; either
     with predefined color names (like with the already available
     'css-color' function), or with hexadecimal color codes prefixed
     with '#'.  All stencils that support a 'color' property now accept
     either a list or a string; in the latter case, that string is used
     directly in the SVG output.  This allows to use alpha transparency
     (entered as "#RRGGBBAA" or "#RGBA") in SVG.

   . PDF bookmarks are now supported and allow for '\tocItem' entries to
     appear in the 'table of contents' panel of PDF viewers that support
     it.

   . '\table-of-contents' now accepts a hierarchical structure;
     '\tocItem' entries may optionally take a symbol (like '\label') or
     a dot-separated list of symbols, indicating their position in the
     score's structure.  A side-effect of that feature is that
     '\tocItem' can no longer take a simple string as its argument; a
     '\markup' command must be used.

   . Using the new options '-dpng-width' and '-dpng-height' it is now
     possible to specify the pixel size of PNG output images.

   . SVG output is now available through the '--svg' command-line option
     (or its canonical form '--format=svg', also shortened as '-fsvg').
     Due to its backend's specificity, that option is not yet compatible
     with other output formats; to get a file in both SVG and PDF, PNG
     or EPS, a second LilyPond run is required.

2.20:

New for musical notation
------------------------

*Displaying pitch improvements*

   . Pitches that have a sharp or flat in their name now need to be
     hyphenated.

     Pitches that contain _double_ sharps or flats in their name,
     however, do not need a second hyphen.

   . Accidental rules can now be defined _across_ 'ChoirStaff' contexts.

   . Two new accidental rules have been added.  Both combine the
     characteristics of 'modern-voice', 'piano' and their equivalents:

     'choral'

     This is the now the default accidental style for 'ChoirStaff'.

     'choral-cautionary'

     The same as 'choral' but with the extra accidentals typeset as
     cautionaries instead.

   . Four new clef glyphs are now available; 'GG' (double-G), 'Tenor G',
     'varC' plus related tessitura and 'Varpercussion'.

   . French note names are now explicitly defined -- previously they were
     aliased to Italian note names.

     Double sharps are entered using an 'x' suffix.

   *Rhythm improvements*

   . Multi-measure rests have length according to their total duration,
     under the control of 'MultiMeasureRest.space-increment'.  Note the
     default value is '2.0'.

   . Improvements to the '\partial' command have been made when used
     with parallel music and/or multiple contexts.

   . It is now possible to change the time signature mid-measure by
     using both the '\time' and '\partial' commands together.

   . Isolated durations in music now stand for unpitched notes.  Pitches
     are taken from the preceding note or chord.  This is especially
     convenient for specifying rhythms in both music and scheme
     functions and can help improve the readability of LilyPond source
     files.

   . Beaming exceptions can now be constructed using the simpler
     '\beamExceptions' scheme function. Note that writing the
     exception pattern without pitches is convenient but not
     mandatory (also see the previous documented rhythm improvement --
     _Isolated durations in music now stand for unpitched notes_.

   . The positioning of tuplet numbers for kneed beams has been
     improved.  Previously, tuplet numbers were placed according to the
     position of the tuplet bracket, even if the bracket was not
     printed.  This could lead to tuplet numbers being 'stranded'.

   . Collision detection for the kneed beam tuplet numbers has also been
     added, shifting the offset horizontally if the number is too close
     to an adjoining note column (but still preserving the number's
     vertical distance).  In the event of a collision -- for example with
     an accidental -- the tuplet number will be shifted vertically
     instead.  If the tuplet number is itself too large to fit within
     the available space, the original, 'bracket-based', positioning
     system will be used instead.

     The original kneed-beam tuplet behavior is still available with a
     new, 'knee-to-beam' property for the 'TupletNumber' layout object.

   *Expressive mark improvements*

   . The ends of hairpins may now be fine-tuned using the 'shorten-pair'
     grob property.  This previously only affected text-spanners (e.g.
     'TupletBracket' and 'OttavaBracket').

     Positive and negative values offset right and left respectively.

   . Individual slurs and phrasing slurs may now be started from an
     explicit note within a chord.

   . A new command '\=X' has been added -- where 'X' can be any
     non-negative integer or symbol -- so that a specific 'id' can be
     assigned to the start and end of slurs and phrasing slurs.

     This is useful when simultaneous slurs are required or if one slur
     overlaps another or when nesting short slurs within a longer one.

   *Repeat notation improvements*

   . The visual style of tremolo slashes (shape, style and slope) is now
     more finely controlled.

   . The music function '\unfoldRepeats' can now take an optional
     argument-list specifying which type(s) of repeated music should be
     unfolded.  Possible entries are 'percent', 'tremolo', 'volta'.  If
     the optional argument-list is unspecified, 'repeated-music' will be
     used, unfolding all.

   *Staff notation improvements*

   . A new command '\magnifyStaff' has been added which scales staff
     sizes, staff lines, bar lines, beamlets and horizontal spacing
     generally at the 'Staff' context level.  Staff lines are prevented
     from being scaled smaller than the default since the thickness of
     stems, slurs, and the like are all based on the staff line
     thickness.

   . A new command '\magnifyMusic' has been added, which allows the
     notation size to be changed without changing the staff size, while
     automatically scaling stems, beams, and horizontal spacing.

   . A new command, '\RemoveAllEmptyStaves', has been made available,
     which acts exactly like '\RemoveEmptyStaves', except for also
     removing empty staves on the first system in a score.

   . A new markup command '\justify-line' has been added.  Similar to
     the '\fill-line' markup command except that instead of setting
     _words_ in columns, the '\justify-line' command balances the
     whitespace between them ensuring that when there are three or more
     words in a markup, the whitespace is always consistent.

   *Editorial annotation improvements*

   . It is now possible to add text to analysis brackets through the
     'HorizontalBracketText' object.

   *Text formatting improvements*

   . Support for making it easier to use alternative 'music' fonts other
     than the default Emmentaler in LilyPond has been added.  See *note
     (lilypond-notation)Replacing the notation font:: for more
     information.

   . Default text fonts have been changed from 'Century Schoolbook L',
     'sans-serif', and 'monospace'.

     For 'svg' backend:
     Family      Default font
     -----------------------------------
     _roman_     'serif'
     _sans_      'sans-serif'
     _typewriter_'monospace'

     'serif', 'sans-serif', and 'monospace' are 'generic-family' in SVG
     and CSS specifications.

     For other backends:
     Family      Default font (alias)   Alias definition lists
     ----------------------------------------------------------------------------
     _roman_     'LilyPond Serif'       TeX Gyre Schola, C059, Century
                                        SchoolBook URW, Century Schoolbook L,
                                        DejaVu Serif, ..., serif
     _sans_      'LilyPond Sans         TeX Gyre Heros, Nimbus Sans, Nimbus
                 Serif'                 Sans L, DejaVu Sans, ..., sans-serif
     _typewriter_'LilyPond Monospace'   TeX Gyre Cursor, Nimbus Mono PS,
                                        Nimbus Mono, Nimbus Mono L, DejaVu
                                        Sans Mono, ..., monospace

     'LilyPond Serif', 'LilyPond Sans Serif', and 'LilyPond Monospace'
     are font aliases defined in the LilyPond dedicated FontConfig
     configuration file '00-lilypond-fonts.conf'.  Where a character
     dosen't exist in the first font listed, the next font listed will
     be used instead for that character.  For details of alias
     definitions, please see to '00-lilypond-fonts.conf' under the
     installed directory.

   . When using OpenType fonts, font features can be used.  Note: Not
     all OpenType fonts have all functions.

   . Two new styles of whiteout are now available.  The 'outline' style
     approximates the contours of a glyph's outline, and its shape is
     produced from multiple displaced copies of the glyph.  The
     'rounded-box' style produces a rounded rectangle shape.  For all
     three styles, including the default 'box' style, the whiteout
     shape's 'thickness', as a multiple of staff-line thickness, can be
     customized.

   . A new markup-command, '\with-dimensions-from', makes
     '\with-dimensions' easier to use by taking the new dimensions from
     a markup object, given as first argument.

   . Markup-command '\draw-squiggle-line' is now available.  Customizing
     is possible with overrides of 'thickness', 'angularity', 'height'
     and 'orientation'

   . Markup-commands '\undertie' and '\overtie' are now available, as
     well as the generic markup-command '\tie'.

New for specialist notation
---------------------------

*Vocal music improvements*

   . A new flexible template suitable for a range of choral music, is
     now provided.  This may be used to create simple choral music, with
     or without piano accompaniment, in two or four staves.  Unlike
     other templates, this template is 'built-in', which means it does
     not need to be copied and edited: instead it is simply '\include''d
     in the input file.  For details, see *note
     (lilypond-learning)Built-in templates::.

   . The '\addlyrics' function now works with arbitrary contexts
     incuding 'Staff'.

   . '\lyricsto' and '\addLyrics' have been 'harmonized'.  Both now
     accept the same kind of delimited argument list that '\lyrics' and
     '\chords' accept.  Backward compatibility has been added so music
     identifiers (i.e.  '\mus') are permitted as arguments.  A
     'convert-ly' rule has been added that removes redundant uses of
     '\lyricmode' and rearranges combinations with context starters such
     that '\lyricsto' in general is applied last (i.e.  like
     '\lyricmode' would be).

   *Unfretted and fretted string instrument improvements*

   . A new note head style for Tabulature has been added --
     'TabNoteHead.style = #'slash'.

   . In fret-diagrams the distance between frets and the distance
     between strings is now independently adjustable.  Available are
     'fret-distance' and 'string-distance' as subproperties of
     'fret-diagram-details'.

   . It is now possible to individually color both the dots and
     parentheses in fret diagrams when using the '\fret-diagram-verbose'
     markup command.

   . Two new properties have been added for use in
     'fret-diagram-details' when using the '\fret-diagram-verbose'
     markup command; 'fret-label-horizontal-offset' which affects the
     'fret-label-indication' and 'paren-padding' which controls the
     space between the dot and the parentheses surrounding it.

   . Additional bass strings (for lute tablature) are supported.

   . String numbers can now also be used to print roman numerals (e.g.
     for unfretted string instruments).

   . 'TabStaff' is now able to print micro-tones for bendings etc.

   *Chord notation improvements*

   . '\chordmode' can now use '< >' and '<< >>' constructs.

   . It is now possible to override the 'text' property of chord names.

New for input and output
------------------------

*Input structure improvements*

   . Blocks introduced with '\header' can be stored in variables and
     used as arguments to music and scheme functions and as the body of
     '#{...#}' constructs.  They are represented as a Guile module.

     While '\book', '\bookpart', '\score', '\with', '\layout', '\midi',
     '\paper' blocks can be passed around in similar manner, they are
     represented by different data types.

   *Titles and header improvements*

   . Page numbers may now be printed in roman numerals, by setting the
     'page-number-type' paper variable.

   *Input file improvements*

   . A new command '\tagGroup' has now been added.  This complements the
     existing '\keepWithTag' and '\removeWithTag' commands.

   *Output improvements*

   . LilyPond source files may now be embedded inside the generated PDF
     files.  This experimental feature is disabled by default and may be
     regarded as unsafe, as PDF documents with hidden content tend to
     present a security risk.  Please note that not all PDF viewers have
     the ability to handle embedded documents (if not, the PDF output
     will appear normally and source files will remain invisible).  This
     feature only works with the PDF backend.

   . The 'output-classic-framework' procedure and the '-dclip-systems'
     are now available with the 'SVG' backend.

   . An argument, '-dcrop', has been added, formatting 'SVG' and 'PDF'
     output without margins or page-breaks.

   . A new 'output-attributes' grob property is now used for svg output
     instead of the 'id' grob property.  It allows multiple attributes
     to be defined as an association list.  For example, '#'((id . 123)
     (class . foo) (data-whatever . "bar"))' will produce the following
     group tag in an SVG file: '<g id="123" class="foo"
     data-whatever="bar"> ... </g>'.

   . The PostScript functionality of stroke adjustment is no longer
     applied automatically but left to the discretion of the PostScript
     device (by default, Ghostscript uses it for resolutions up to
     150dpi when generating raster images).  When it is enabled, a more
     complex drawing algorithm designed to benefit from stroke
     adjustment is employed mostly for stems and bar lines.

     Stroke adjustment can be forced by specifying the command line
     option '-dstrokeadjust' to LilyPond.  When generating 'PDF' files,
     this will usually result in markedly better looking 'PDF' previews
     but significantly larger file size.  Print quality at high
     resolutions will be unaffected.

   . Added a new 'make-path-stencil' function that supports all 'path'
     commands both relative and absolute:

     'lineto', 'rlineto', 'curveto', 'rcurveto', 'moveto', 'rmoveto',
     'closepath'.  The function also supports 'single-letter' syntax
     used in standard SVG path commands:

     'L', 'l', 'C', 'c', 'M', 'm', 'Z' and 'z'.  The new command is also
     backward-compatible with the original 'make-connected-path-stencil'
     function.  Also see 'scm/stencil.scm'.

   *MIDI improvements*

   . The most common articulations are now reflected in MIDI output.
     Accent and marcato make notes louder; staccato, staccatissimo and
     portato make them shorter.  Breath marks shorten the previous note.

     This behavior is customizable through the 'midiLength' and
     'midiExtraVelocity' properties on 'ArticulationEvent'.  See
     'script-init.ly' for examples.

   . Improved MIDI output for breathe marks.  After tied notes, breaths
     take time _only_ from the last note of the tie; e.g.  '{ c4~ c8
     \breathe }' performs as '{ c4~ c16 r }' instead of '{ c4 r8 }'.
     This is more consistent with articulations and how humans interpret
     breaths after ties.  It now also makes it easier to align
     simultaneous breathe marks over multiple parts, all with different
     note lengths.

   . There is now support for controlling the 'expression level' of MIDI
     channels using the 'Staff.midiExpression' context property.  This
     can be used to alter the perceived volume of even sustained notes
     (albeit in a very 'low-level' way) and accepts a number value
     between '0.0' and '1.0'.

   . When outputting MIDI, LilyPond will now store the 'title' defined
     in a score's '\header' block (or, if there is no such definition on
     the '\score' level, the first such definition found in a '\header'
     block of the score's enclosing '\bookpart', '\book', or top-level
     scope) as the name of the MIDI sequence in the MIDI file.
     Optionally, the name of the MIDI sequence can be overridden using
     the new 'midititle' '\header' field independently of 'title' (for
     example, in case 'title' contains markup code which does not render
     as plain text in a satisfactory way automatically).

   . Support for making it easier to use alternative 'music' fonts other
     than the default Emmentaler in LilyPond has been added.  See *note
     (lilypond-notation)Replacing the notation font:: for more
     information.

   *Extracting music improvements*

   . '\displayLilyMusic' and its underlying Scheme functions no longer
     omit redundant note durations.  This makes it easier to reliably
     recognize and format standalone durations in expressions like
          { c4 d4 8 }

New for spacing issues
----------------------

*Page breaking improvements*

   . There are two new page breaking functions.  'ly:one-page-breaking'
     automatically adjusts the height of the page to fit the music, so
     that everything fits on one page.
     'ly:one-line-auto-height-breaking' is like 'ly:one-line-breaking',
     placing the music on a single line and adjusting the page width
     accordingly, however it also automatically adjusts the page height
     to fit the music.

   *Vertical and Horizontal spacing improvements*

   . It is now possible to move systems with reference to their current
     position using the 'extra-offset' subproperty of
     'NonMusicalPaperColumn.line-break-system-details'.  Both vertical
     and horizontal changes are possible.  This feature is especially
     useful for making slight adjustments to the default vertical
     position of individual systems.  See *note
     (lilypond-notation)Explicit staff and system positioning:: for more
     information.

   . Improved visual spacing of small and regular 'MI' Funk and Walker
     noteheads so they are now the same width as other shaped notes in
     their respective sets.  'SOL' noteheads are also now visually
     improved when used with both the normal Aiken and Sacred Harp
     heads, as well as with the thin variants.

   . 'LeftEdge' now has a definable 'Y-extent' (i.e.vertical).  See
     *note (lilypond-internals)LeftEdge::.

   . Grobs and their parents can now be aligned separately allowing more
     flexibility for grob positions.  For example the 'left' edge of a
     grob can now be aligned on the 'center' of its parent.

   . Improved horizontal alignment when using 'TextScript', with
     'DynamicText' or 'LyricText'.

New for changing defaults
-------------------------

     An optional argument for the '\afterGrace' command has been added.

     '\afterGrace' now has an optional argument to specificy the spacing
     fraction position of its notes.

   . All of '\override', '\revert', '\set', and '\unset' now work with
     the '\once' prefix for making one-time settings.

New for Internal interfaces and functions
-----------------------------------------

   . The music and grob property 'spanner-id', used for distinguishing
     simultaneous slurs and phrasing slurs, has been changed from a
     string to a _key_ which can be either a non-negative integer or
     symbol (also see the previous documented expressive mark
     improvement -- _A new command \=X has been added_).

   . Context properties named in the 'alternativeRestores' property are
     restored to their value at the start of the _first_ alternative in
     all subsequent alternatives.

   . LilyPond functions defined with 'define-music-function',
     'define-event-function', 'define-scheme-function' and
     'define-void-function' can now be directly called from Scheme as if
     they were genuine Scheme procedures.  Argument checking and
     matching will still be performed in the same manner as when calling
     the function through LilyPond input.  This includes the insertion
     of defaults for optional arguments not matching their predicates.
     Instead of using '\default' in the actual argument list for
     explicitly skipping a sequence of optional arguments,
     '*unspecified*' can be employed.

   . Current input location and parser are now stored in GUILE fluids
     and can be referenced via the function calls '(*location*)' and
     '(*parser*)'.  Consequently, a lot of functions previously taking
     an explicit 'parser' argument no longer do so.

     Functions defined with 'define-music-function',
     'define-event-function', 'define-scheme-function' and
     'define-void-function' no longer use 'parser' and 'location'
     arguments.

     With those particular definitions, LilyPond will try to recognize
     legacy use of 'parser' and 'location' arguments, providing
     backwards-compatible semantics for some time.

   . Scheme functions and identifiers can now be used as output
     definitions.

   . Scheme expressions can now be used as chord constituents.

   . Music (and scheme and void) functions and markup commands that just
     supply the final parameters to a chain of overrides, music function
     and markup command calls can now be defined in the form of just
     writing the expression cut short with '\etc'.

   . Dot-separated symbol lists like 'FretBoard.stencil' were already
     supported as of version 2.18.  They may now also contain unsigned
     integers, and may alternatively be separated by commata.

   . Such lists may also be used in expressions for assignments, sets,
     and overrides.

   . Association list elements could previously be assigned values
     individually (for example, paper variables like
     'system-system-spacing.basic-distance').

     In combination with the previously mentioned changes, this allows
     setting and referencing pseudovariables like 'violin.1'.

   . The markup-list-command '\table' is now available.  Each column may
     be aligned differently.
   . 'InstrumentName' now supports 'text-interface'.

   . The 'thin-kern' property of the 'BarLine' grob has been renamed to
     'segno-kern'.

   . 'KeyCancellation' grobs now ignore cue clefs (like 'KeySignature'
     grobs do).

   . Add support for '\once \unset'