anchor.diff:  
   (defmethod object-ast ((ast rplacd-ast))
     (second (children ast)))
  +
  +;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  +;;;
  +;;; Class FIXNUM-ADD-AST.
  +;;;
  +;;; This AST can be used to implement a binary addition function.  It
  +;;; requires both its arguments to be of type FIXNUM.  It generates
  +;;; two values that we may call RESULT (a FIXNUM) and OVERFLOWP (a
  +;;; Boolean).  OVERFLOWP indicates whether the operation resulted in
  +;;; an overflow.  When it is false, then there was no overflow, and
  +;;; RESULT is the sum of the two arguments.  When OVERFLOWP is true,
  +;;; the operation resulted in an overflow.  Then, if RESULT is
  +;;; negative, then a BIGNUM with the value 2^n + RESULT should be
  +;;; created, where n is the number of bits in a word.  If RESULT is
  +;;; non-negative, then a BIGNUM with the value RESULT - 2^n should be
  +;;; created.
  +
  +(defclass fixnum-add-ast (ast)
  +  ())
  +
  +(defun make-fixnum-add-ast (arg1-ast arg2-ast)
  +  (make-instance 'fixnum-add-ast
  +    :children (list arg1-ast arg2-ast)))
  
positive.code: 
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  ;;; Class RPLACA-AST
  ;;;
  ;;; This AST can be used to implement the function RPLACA and the
  ;;; function (SETF CAR) in implementations where it is a function.
  
negative.code: 
  ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
  ;;;
  ;;; Class RPLACD-AST.
  ;;;
  ;;; This AST can be used to implement the function RPLACD and the
  ;;; function (SETF CDR) in implementations where it is a function.
  