(defmacro reducing (rf &body body)
  (destructuring-bind ((&rest b0) (a1 &rest b1) (a2 &rest b2)) body
    (with-gensyms (result input resultp inputp)
      `(flet (,(when (typep rf '(or symbol list))
                     `(,rf (&rest args) (apply ,rf args))))
         (lambda (&optional (,result nil ,resultp) (,input nil ,inputp))
           (declare (optimize speed (debug 0) (safety 0)))
           (block nil
             (cond (,inputp (let ((,(car a2) ,result)
                                  (,(cadr a2) ,input))
                              ,@b2))
                   (,resultp (let ((,(car a1) ,result))
                               ,@b1))
                   (t ,@b0))))))))