The special form interactive turns a Lisp function into a
command. The interactive form must be located at top-level in
the function body (usually as the first form in the body), or in the
interactive-form property of the function symbol. When the
interactive form is located in the function body, it does
nothing when actually executed. Its presence serves as a flag, which
tells the Emacs command loop that the function can be called
interactively. The argument of the interactive form controls
the reading of arguments for an interactive call.