Each predicate of the program is assigned a module, called it's definition module. The definition module of a predicate is always the module in which the predicate was originally defined. Each active goal in the Prolog system has a context module assigned to it.
The context module is used to find predicates from a Prolog term. By default, this module is the definition module of the predicate running the goal. For meta-predicates however, this is the context module of the goal that invoked them. We call this module_transparent in SWI-Prolog. In the `using maplist' example above, the predicate maplist/3 is declared module_transparent. This implies the context module remains extend, the context module of add_extension/3. This way maplist/3 can decide to call extend_atom in module extend rather than in it's own definition module.
All built-in predicates that refer to predicates via a Prolog term are declared module_transparent. Below is the code defining maplist.
:- module(maplist, maplist/3). :- module_transparent maplist/3. % maplist(+Goal, +List1, ?List2) % True if Goal can succesfully be applied to all succesive pairs % of elements of List1 and List2. maplist(_, [], []). maplist(Goal, [Elem1|Tail1], [Elem2|Tail2]) :- apply(Goal, [Elem1, Elem2]), maplist(Goal, Tail1, Tail2).