anchor.diff:                     `(&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)))
  +           ,body)))))
  
positive.code:                   (let ((splice (splice (cdr f) env)))
                      `(multiple-value-call (function ,fn)
                         ,@splice)))))
               (walk-op (x env)
                 (declare (ignorable env))
                 #+sbcl
  
negative.code:                      ((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)))
  