(defmacro with-output-to-stream-or-string
    ((symbol &optional (stream-or-null symbol)) &body body)
  "Bind `symbol` to an output stream, run `body`, and return appropriately.

  If `stream-or-null` is a stream, `symbol` will be bound to it and nothing will
  be returned.

  If `stream-or-null` is `nil`, `symbol` will be bound to a string output stream
  and the resulting string will be returned.

  "
  (let ((want-string (gensym "WANT-STRING")))
    `(let* ((,symbol ,stream-or-null)
            (,want-string (null ,symbol))
            (,symbol (or ,symbol (make-string-output-stream))))
       ,@body
       (if ,want-string
         (get-output-stream-string ,symbol)
         (values)))))