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\subsection{library(explain): Describe Prolog Terms}

\label{sec:explain}

The \file{library(explain)} describes prolog-terms. The most useful
functionality is its cross-referencing function.

\begin{code}
?- explain(subset(_,_)).
"subset(_, _)" is a compound term
    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
    /staff/jan/lib/pl-5.6.17/library/lists.pl:307
    Referenced from 2-th clause of lists:subset/2
    Possibly referenced from 2-th clause of lists:subset/2
\end{code}

Note that PceEmacs can jump to definitions and \predref{gxref}{0} can be used for
an overview of dependencies.\vspace{0.7cm}

\begin{description}
    \predicate[det]{explain}{1}{@Term}
Give an explanation on \arg{Term}. The argument may be any Prolog data
object. If the argument is an atom, a term of the form \verb$Name/Arity$
or a term of the form \verb$Module:Name/Arity$, \predref{explain}{1} describes the
predicate as well as possible references to it. See also \predref{gxref}{0}.

    \predicate[nondet]{explain}{2}{@Term, -Explanation}
True when \arg{Explanation} is an explanation of \arg{Term}. The explaination is
a list of elements that is printed using \verb$print_message(information, explain(Explanation))$.
\end{description}