The predicate goal_expansion/2 is first called in the module that is being compiled, and then follows the module inheritance path as defined by default_module/2, i.e., by default user and system. If Goal is of the form Module:Goal where Module is instantiated, goal_expansion/2 is called on Goal using rules from module Module followed by default modules for Module.

Only goals appearing in the body of clauses when reading a source file are expanded using this mechanism, and only if they appear literally in the clause, or as an argument to a defined meta-predicate that is annotated using‘0' (see meta_predicate/1). Other cases need a real predicate definition.

The expansion hook can use prolog_load_context/2 to obtain information about the context in which the goal is expanded such as the module, variable names or the encapsulating term.