prolog-underscore-wordchar-flag is a variable defined in `prolog.el'.
Its value is
nil
This variable is obsolete since 24.4;
use `superword-mode' instead.
Documentation:
Non-nil means underscore (_) is a word-constituent character.
You can customize this variable.
This variable was introduced, or its default value was changed, in
version 24.1 of Emacs.