anchor.diff:                       ((placeholder? x env) (make-var))
                        ((numbered-placeholder? x env)
                         (make-var/numbered x))
  +                     ((op? x)
  +                      (warn-nested-op)
  +                      x)
                        ((listp x)
                         (let ((splice? (some (rcurry #'rest-placeholder? env) x))
                               (y (mapcar (rcurry #'walk-op env) x)))
  
positive.code:                    `(&rest ,(intern (string '_*))))))
          `(lambda (,@(reverse vars) ,@rest)
             (declare (ignorable ,@vars))
             ,(forbid-nested-op body))))))
  
  (defun forbid-nested-op (form)
    `(locally (declare #+sbcl (sb-ext:disable-package-locks op))
       (macrolet ((op (&rest args)
                    (declare (ignore args))
                    (warn "The OP macro cannot be nested.")))
         (declare #+sbcl (sb-ext:enable-package-locks op))
         ,form)))
  
negative.code:                          (values (make-var) t))
                          ((numbered-placeholder? f e)
                           (values (make-var/numbered f) t))
                          ((op? f)
                           (warn-nested-op)
                           (values f t))
                          ((and (listp f) (some (lambda (x) (rest-placeholder? x e)) f))
                           (let ((f (cons (car f)
                                          (mapcar (lambda (x) (walk-op x e))
  