Log Message:
Update to Racket 6.3

Changes in Racket 6.3

- Racket's macro expander uses a new representation of binding called
  "set of scopes". The new binding model provides a simpler
  explanation of how macros preserve binding, especially across module
  boundaries and in hygiene-bending expansions. The new expander is
  mostly compatible with existing Racket macros, but there are some
  incompatibilities. For the formally inclined, a research paper on
  this macro system will appear at POPL next year:

      http://www.cs.utah.edu/plt/scope-sets/

- Racket's GUI library now uses Gtk+ 3 when available, instead of Gtk+ 2.
  Set the `PLT_GTK2` environment variable to select Gtk+ 2.

- Added a new Redex tutorial based on a week-long workshop in SLC.

- Better syntax error checking for Redex patterns that do not use
  holes correctly.

- The blueboxes are more agressive about finding names to look up in
  the docs, meaning they are useful much more often.

- Submodules are now fully supported in Typed Racket. Previously, some
  uses of submodules would produce internal errors, making it hard to
  `module+ test` and `module+ main` effectively in Typed Racket. The
  switch to the set-of-scopes expander fixed these problems, and
  submodules are now happily at home in Typed Racket.

- The `typed/racket/unsafe` library provides import and export forms
  that circumvent contract generation. This improves performance for
  typed-untyped interaction at the cost of safety and debuggability.

- Typed Racket provides experimental support for units (from
  `racket/unit`).

- The experimental `define-new-subtype` form allows overlaying finer
  distinctions between otherwise identical types, similar to Haskell's
  `newtype`.

- The `Promise` type constructor changes in a backwards-incompatible
  way to exclude promises created with `promise/name`.

- The `unstable-*` packages are out of the main distribution. Most of
  their contents have been either merged with established Racket
  libraries or spun off as their own packages. This change is
  backwards compatible for packages that properly list their
  dependencies. Full details:
  http://blog.racket-lang.org/2015/10/retiring-unstable.html

- edu: `big-bang` supports a display-mode clause so that world
  programs can take over the entire screen.

Changes in Racket 6.2.1

- For the How to Design Programs teaching languages, DrRacket offers
  an option to use the old style for printing the constants `true`,
  `false`, and `empty` instead of `#true`, `#false`, and `'()`.

- The teaching languages come with some additional functions to match
  the August 2015 stable release of HtDP 2nd edition.

- A repair to the compiler avoids an infinite loop at compile time for
  certain expressions that should loop forever at run time.