Version 4.2.1, July 2009
  Added string->unreadable-symbol and symbol-interned?
  Added syntax-local-lift-provide
  Added syntax-local-lift-values-expression
  Added identifier-prune-lexical-context and quote-syntax/prune

  Version 4.2, May 2009
  Changed visiting of modules at phase N to happen only when compilation
   at phase N starts
  Changed expander to detect a reaname transformer and install a
   free-identifier=? syntax-object equivalence
  Changed provide to convert an exported rename transformer to its
   free-identifier=? target
  Added 'not-free-identifier=? syntax property to disable free-identifier=? 
   propagation through a rename transformer
  Added prop:rename-transformer and prop:set!-transformer
  Fixed scheme/local so that local syntax bindings are visible to later
   local definitions
  Changed current-process-milliseconds to accept a thread argument
  Added hash-hash-key?, hash-ref!
  Added in-sequences, in-cycle

  Version 4.1.5, March 2009
  Allow infix notation for prefab structure literals
  Change quasiquote so that unquote works in value positions of #hash
  Change read-syntax to represent #hash value forms as syntax
  Added bitwise-bit-field

  Version 4.1.4, January 2009
  Changed memory accounting to bias charges to parent instead of children
  Changed function contracts to preserve tail recursion in many cases
  Added scheme/package, scheme/splicing, ffi/objc
  Added exception-backtrace support for x86_84+JIT
  Added equal?/recur
  Added equal<%> and interface* to scheme/class
  Added procedure-rename
  Added extra arguments to call-with-continuation-prompt
  Added extra argument to raise-syntax-error
  Added compile-context-preservation-enabled
  Added syntax-local-lift-require
  Added internal-definition-context-seal, which must be used on an
   internal-definition context before it's part of a fully expanded form
  Added syntax-local-make-delta-introducer
  Changed make-rename-transformer to accept an introducer argument that
   cooperates with syntax-local-make-delta-introducer
  Added internal-defininition-context?
  Added identifier-remove-from-defininition-context

  Version 4.1.3, November 2008
  Changed scheme to re-export scheme/port
  In scheme/port: added 
  Added call-with-immediate-continuation-mark
  In scheme/port: added port->string, port->bytes, port->lines
   port->bytes-lines, display-lines, [call-]with-input-from-{string,bytes},
   and [call-]with-output-to-{string,bytes}
  In scheme/file: added file->string, file->bytes, file->lines,
   file->value, file->bytes-lines, write-to-file, display-to-file,
   and display-lines-to-file
  Added variable-reference? and empty #%variable-reference form
  Extended continuation-marks to work on a thread argument