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\section{library(varnumbers): Utilities for numbered terms}

\label{sec:varnumbers}

\begin{tags}
    \tag{See also}
\predref{numbervars}{4}, \predref{\Sstructeq}{2} (\predref{variant}{2}).
    \tag{Compatibility}
This library was introduced by Quintus and available in
many related implementations, although not with exactly the
same set of predicates.
\end{tags}

This library provides the inverse functionality of the built-in
\predref{numbervars}{3}. Note that this library suffers from the known issues that
'\$VAR'(X) is a normal Prolog term and, -unlike the built-in numbervars-,
the inverse predicates do \textit{not} process cyclic terms. The following
predicate is true for any acyclic term that contains no '\$VAR'(X),
\verb$integer(X)$ terms and no constraint variables:

\begin{code}
always_true(X) :-
      copy_term(X, X2),
      numbervars(X),
      varnumbers(X, Copy),
      Copy =@= X2.
\end{code}

\vspace{0.7cm}

\begin{description}
    \predicate[det]{numbervars}{1}{+Term}
Number variables in \arg{Term} using \$VAR(N). Equivalent to
\verb$numbervars(Term, 0, _)$.

\begin{tags}
    \tag{See also}
\predref{numbervars}{3}, \predref{numbervars}{4}
\end{tags}

    \predicate[det]{varnumbers}{2}{+Term, -Copy}
Inverse of \predref{numbervars}{1}. Equivalent to \verb$varnumbers(Term, 0, Copy)$.

    \predicate[det]{varnumbers}{3}{+Term, +Start, -Copy}
Inverse of \predref{numbervars}{3}. True when \arg{Copy} is a copy of \arg{Term} with
all variables numbered \Sge{} \arg{Start} consistently replaced by fresh
variables. Variables in \arg{Term} are \textit{shared} with \arg{Copy} rather than
replaced by fresh variables.

\begin{tags}
    \tag{Errors}
\verb$domain_error(acyclic_term, Term)$ if \arg{Term} is cyclic.
    \tag{Compatibility}
Quintus, SICStus. Not in YAP version of this library
\end{tags}

    \predicate[det]{max_var_number}{3}{+Term, +Start, -Max}
True when \arg{Max} is the max of \arg{Start} and the highest numbered
\$VAR(N) term.

\begin{tags}
    \tag{author}
Vitor Santos Costa
    \tag{Compatibility}
YAP
\end{tags}

    \predicate[det]{varnumbers_names}{3}{+Term, -Copy, -VariableNames}
If \arg{Term} is a term with numbered and named variables using the
reserved term '\$VAR'(X), \arg{Copy} is a copy of \arg{Term} where each
'\$VAR'(X) is consistently replaced by a fresh variable and
Bindings is a list \verb$X = Var$, relating the \arg{X} terms with the
variable it is mapped to.

\begin{tags}
    \tag{See also}
\predref{numbervars}{3}, \predref{varnumbers}{3}, \predref{read_term}{3} using the
\verb$variable_names$ option.
\end{tags}
\end{description}