anchor.diff:  (defmacro define-modf-function (name nth-arg (new-val &rest args) &body body)
     "Define a new modf function.  It inverts NAME forms by modifying the NTH-ARG
   term of the arguments of the place form in the MODF macro."
  +  ;; Side effect in a macro, I know.  How can you do this via EVAL-WHEN if I
  +  ;; want it to run even if not a toplevel macro?
  +  (setf (gethash name *modf-nth-arg*)
  +        nth-arg )
     `(progn
        (eval-when (:compile-toplevel :load-toplevel :execute)
          (setf (gethash ',name *modf-nth-arg*)
  
positive.code:   "Define a new modf method.  It inverts NAME forms by modifying the NTH-ARG
  term of the arguments of the place form in the MODF macro.  This method can
  specialize on any of ARGS."
    ;; Side effect in a macro, I know.  How can you do this via EVAL-WHEN if the
    ;; rest of the macro-expansion depends on the side effect.
    `(progn
       (eval-when (:compile-toplevel :load-toplevel :execute)
         (setf (gethash ',name *modf-nth-arg*)
  
negative.code: should return an expansion from EXPR to a form that will build a new object that
  has NEW-VAL in the place specified by expr.  NTH-ARG marks which argument is
  considered the actual data which will be inverted next."
    (setf
     (gethash name *modf-nth-arg*)
     nth-arg )
    `(eval-when (:compile-toplevel :load-toplevel :execute)
       (setf
        (gethash ',name *modf-nth-arg*)
  