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explain.pl -- Describe Prolog Terms
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The library(explain) describes prolog-terms. The most useful functionality is its cross-referencing function.

?- explain(subset(_,_)).
"subset(_, _)" is a compound term
        Referenced from 2-th clause of lists:subset/2
        Referenced from 46-th clause of prolog_xref:imported/3
        Referenced from 68-th clause of prolog_xref:imported/3
lists:subset/2 is a predicate defined in
        Referenced from 2-th clause of lists:subset/2
        Possibly referenced from 2-th clause of lists:subset/2

Note that the help-tool for XPCE provides a nice graphical cross-referencer.

Source explain(@Term) is det
Give an explanation on Term. The argument may be any Prolog data object. If the argument is an atom, a term of the form Name/Arity or a term of the form Module:Name/Arity, explain/1 describes the predicate as well as possible references to it. See also gxref/0.
Source explain(@Term, -Explanation) is nondet
True when Explanation is an explanation of Term.
Source known_predicate(:Head)[private]
Succeeds if we know anything about this predicate. Undefined predicates are considered `known' for this purpose, so we can provide referenced messages on them.
Source explain_predicate(:Head, -Explanation) is det[private]