How to define a (derived) minor mode inaccessible to the user

[Marcin Borkowski]

Hello everyone,

I'd like to define a special-purpose major mode, derived from
`special-mode', in such a way that the user cannot set it using M-x.
(It is to be called via another command.)  It seems that
`define-derived-mode' always sets the mode as `interactive'.  I tried to
define my mode using

(define-derived-mode
  my--mode
  special-mode
  "My mode")

and then did

M-: (symbol-plist 'my--mode)

expecting an `interactive-form' entry on the plist - but it was not
there.  (I wanted to remove it from the list so that it can't be called
with M-x.)

What's going on?  Is it possible to define a (derived) major mode so
that it can't be called interactively?

TIA,

--
Marcin Borkowski
http://mbork.pl