anchor.diff:                    (let ((splice (splice (cdr f) env)))
                       `(multiple-value-call (function ,fn)
                          ,@splice)))))
  +             (op? (form)
  +               (match form
  +                 ((list* 'op _) t)))
  +             (warn-nested-op ()
  +               (warn "The ~s macro cannot be nested." 'op))
                (walk-op (x env)
                  (declare (ignorable env))
                  #+sbcl
  
positive.code:                      ((placeholder? x env) (make-var))
                       ((numbered-placeholder? x env)
                        (make-var/numbered x))
                       ((listp x)
                        (let ((splice? (some (rcurry #'rest-placeholder? env) x))
                              (y (mapcar (rcurry #'walk-op env) x)))
  
negative.code:                    `(&rest ,(intern (string '_*))))))
          `(lambda (,@(reverse vars) ,@rest)
             (declare (ignorable ,@vars))
             ,body)))))
  