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Use of Term Expansion

This example illustrates the use of user:term_expansion/[2,4] to augment the built-in predicate expand_term/2 which works as a filter on the input to compile and consult. The code below will allow the declaration :- wait f/3 as an alias for :- block f(-,?,?). Wait declarations were used in previous versions of SICStus Prolog.

Note the multifile declaration, which prevents this user:term_expansion/[2,4] clause from erasing any other clauses for the same predicate that might have been loaded. 

:- op(1150, fx, [wait]).

:- multifile user:term_expansion/2.
user:term_expansion((:- wait F/N), (:- block Head)) :-
        functor(Head, F, N),
        wb_args(N, Head).

wb_args(0, _Head).
wb_args(1, Head) :- arg(1, Head, -).
wb_args(N, Head) :-
        N>1,
        arg(N, Head, ?),
        N1 is N-1,
        wb_args(N1, Head).