anchor.diff:     ;; elements FROM, TO, and BY in the order that they were given in
      ;; the clause.
      (%order :initarg :order :reader order)
  -   (%form-value-var :initform (gensym) :reader form-value-var)
      ;; The form that was given after one of the LOOP keywords FROM,
      ;; UPFROM, or DOWNFROM, or 0 if none of these LOOP keywords was
      ;; given.
  
positive.code: 		   :start-form from
  		   :end-form (cdr to)
  		   :by-form by
  		   :termination-test (first to)))
  	       'simple-var-parser
  	       'optional-type-spec-parser
  	       (alternative 'to-parser 'downto-parser 'above-parser)
  
negative.code: 		   :start-form from
  		   :end-form (cdr to)
  		   :by-form by
  		   :termination-test (car to)))
  	       'simple-var-parser
  	       'optional-type-spec-parser
  	       'by-parser
  