* Advice package now signals error if the user attempts to advise

* Advice package now signals error if the user attempts to advise
  anything but an compound procedure.

* Continuation parser keeps track of the type of the previous stack
  frame.  This information is used as context in some situations.
  This mechanism replaces special-purpose flag `allow-next-extended?'.

* Continuation parser slightly reorganized and commented to make it
  easier to understand.

* Debugger modified to provide more flexible control over use of
  history information, to provide more detailed information about
  stack frames, and to make it more self-explanatory.  Also knows
  about "simulated" compiled-code environment frames, and ignores
  them.

* The environment inspector has been modified to make it more
  self-explanatory.  The N command has been replaced by an O command
  like that of the debugger.

* `pretty-print' now has additional optional argument that specifies
  an indentation for the printed expression.  If given, the output is
  indented by that many columns.

* The emacs interface now has a hook for evaluating arbitrary
  emacs-lisp expressions.  This is used to provide a better debugger
  interface.

  **** This requires "xscheme.el" version 1.26 or later. ****

* `stack-frame/debugging-info' now returns a third value,
  "subexpression", which indicates the subexpression of the expression
  that the next later subproblem is evaluating.

* The lambda abstraction now forces the use of internal lambda
  expressions for auxiliary variables.  This is required for correct
  semantics of `letrec'.

* `make-lambda' now does error-checking on its parameter-list
  arguments, which disallows duplicates in the parameter lists.

* The `procedure' abstraction has been split off into a separate file.
  `procedure-arity' has been modified to handle entities correctly.
  A new datatype, `apply-hook', is like entities except that it
  doesn't pass itself to the handler.  `compound-procedure' operations
  have been removed from the global environment; use generic
  operations instead.

* The unsyntaxer has a new entry point, `unsyntax-with-substitutions',
  which allows subexpressions of an expression to be replaced in the
  output with arbitrary objects.

* Removed `dynamic-state-let' from `system-global-syntax-table'.

* The syntaxer now disallows the use of syntactic keywords as
  variables.  This applies to references, bindings, and definitions.

* The syntaxer signals an error if the name of a named `let' is also
  one of its bound variables.

* The syntaxer signals an error if there are duplicates in the
  parameters of a lambda expression.

* Compiled-code environments that do not have interpreter-compatible
  ancestors now simulate such ancestors for debugging convenience.
  The simulated ancestor is the closing environment of the compiled
  code, if known, otherwise it is the system global environment.