prolog-imenu-max-lines

prolog-imenu-max-lines is a variable defined in `prolog.el'.
Its value is

3000

Documentation:
The maximum number of lines of the file for imenu to be enabled.
Relevant only when `prolog-imenu-flag' is non-nil.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.