Log Message:
Update pmw to 4.51

Many changes, including:

. Implemented &&& continuation lines, mainly for new replicator (see \ 
next).

. Implemented &* replication feature.

. Added escapes \<< and \>> for double typographic quotes.

. Implemented bracketed noteheads facility.

. Implemented key N.

. Transposition values of multiples of 11 or 12 caused PMW to give up with a
  hard internal error for some note values.

. A maximum (accumulated) transposition value of 60 is now implemented. This
  is 5 octaves, which should be enough for anybody.

. Added \nh\ and \nc\ so that individual notes can have noteheads set as
  harmonic or cross. This makes it possible to have chords containing
  different noteheads.

. Added /cx, /llc, /lrc, /rlc, and /rrc to slurs and lines to allow for
  horizontal positioning by musical offset a la MusicXML.

. Added Unicode support for non-standardly encoded PostScript fonts via .utr
  files.

. Added new "rest" letter T, the crotchet version giving a single \ 
slash beat
  indicator and the whole bar rest version giving a "percent" \ 
repetition sign.