anchor.diff:  ;;; Each AST is compiled in a particular COMPILATION CONTEXT or
   ;;; CONTEXT for short.  A context object has three components:
   ;;;
  -;;; 1. SUCCESSORS, which is a proper list containing zero, one or two
  +;;; 1. SUCCESSORS, which is a proper list containing zero or two
   ;;; elements.  These elements are instructions resulting from the
   ;;; generation of the code that should be executed AFTER the code
   ;;; generated from this AST.  If the list contains two elements, then
  
positive.code: ;;;
  ;;; The following combinations of SUCCESSORS and RESULTS can occur:
  ;;;
  ;;;   SUCCESSORS is the empty list.  Then RESULTS is T, which means
  ;;;   that ALL the values are required.  Forms that are in TAIL
  ;;;   POSITION are compiled in a context like this.
  ;;;
  ;;;   SUCCESSORS has one element.  then RESULTS can be a list or T.
  ;;;
  ;;;      If RESULTS is the empty list, this means that no values are
  ;;;      required.  Forms inside a PROGN other than the last are
  
negative.code: ;;; required.  In this case, the first element of the list is the
  ;;; successor to use when the value generated by the AST is NIL, and
  ;;; the second element is the successor to use when the value
  ;;; generated by the AST is something other than NIL.
  ;;;
  ;;; 2. RESULTS, indicating how many values are required from the
  ;;; compilation of this AST.  It can be either a list or an atom as
  ;;; describe below.  If it is a list, it contains zero or more lexical
  ;;; locations into which the generated code must put the values of
  ;;; this AST.  If the list contains more elements than the number of
  ;;; values generated by this AST, then the remaining lexical locations
  ;;; in the list must be filled with NIL by the code generated from
  ;;; this AST.  If it is an atom then it is a single datum of the type
  ;;; VALUES-LOCATION, which means that ALL values are required and
  ;;; should be stored in that location.
  ;;;
  ;;; 3. INVOCATION, an ENTER-INSTRUCTION or NIL.  When it is an
  ;;; ENTER-INSTRUCTION, it indicates the function to which the code to
  