[m-dev.] :- external directive

[James Goddard]

Hello out there,

Can anyone explain to me what the "external" directive
does in the following code fragment?  It doesn't
appear to be in the documentation.

(from library/builtin.m:431)

%-----------------------------------------------------------------------------%

:- external(unify/2).
:- external(compare/3).
:- external(compare_representation/3).

ordering(X, Y) = R :-
        compare(R, X, Y).

        % simplify__goal automatically inlines these
definitions.
        %
X  @< Y :- compare((<), X, Y).
X @=< Y :- not compare((>), X, Y).
X @>  Y :- compare((>), X, Y).
X @>= Y :- not compare((<), X, Y).

%-----------------------------------------------------------------------------%

Cheers,

James

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