🚀 go-pugleaf

In an emacs 18.55.0 running under Berkeley UNIX, I defined

(defun insert-many-instances (format-string how-many)
  (interactive "sformat to make string (%d must appear once): \np")
  (while (> how-many 0)
    (insert (format format-string current-gensym-number))
    (setq current-gensym-number (1+ current-gensym-number))))

: when i then do C-U M-x insert-many-instances i get an error that backtraces
thus: 

Signalling: (error "Format string wants too many arguments")
  call-interactively(insert-many-instances)
  (let ...)
  execute-extended-command(insert-many-instances (4))
* call-interactively(execute-extended-command)


If i replace the interactive string with
  "sformat to make string (%%d must appear once): \np"
[note the doubled %] it works just as i intend; the prompt only shows one %.

The documentation for interactive makes no mention of the string arg being a
format string.

Also, the documentation for format doesn't mention the use of a double % to get
a % into the output.

-dk