If working with an application split into several modules, it is often useful to let files begin with a "mode line":
%%% -*- Mode: Prolog; Module: ModuleName; -*-
The Emacs interface will look for the mode line and notify the SICStus
Prolog module system that code fragments being incrementally
reconsulted or recompiled should be imported into the module
ModuleName. If the mode line is missing, the code fragment will
be imported into the type-in module. An additional benefit of the
mode line is that it tells Emacs that the file contains Prolog code,
regardless of the setting of the Emacs variable
auto-mode-alist. A mode line can be inserted by choosing
Insert/Module modeline in the Prolog menu.