SWI-Prolog 7.4.2 Reference Manual: Section A.38

## A.38 library(varnumbers): Utilities for numbered terms

See also
  numbervars/4, =@=/2 (variant/2).
Compatibility
  This library was introduced by Quintus and available in many related implementations, although not with exactly the same set of predicates.

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

    always_true(X) :-
          copy_term(X, X2),
          numbervars(X),
          varnumbers(X, Copy),
          Copy =@= X2.

[det]numbervars(+Term)
  Number variables in Term using $VAR(N). Equivalent to `numbervars(Term, 0, _)`.
  See also
    numbervars/3, numbervars/4
[det]varnumbers(+Term, -Copy)
  Inverse of numbervars/1. Equivalent to `varnumbers(Term, 0, Copy)`.
[det]varnumbers(+Term, +Start, -Copy)
  Inverse of numbervars/3. True when Copy is a copy of Term with all variables numbered `>=` Start consistently replaced by fresh variables. Variables in Term are shared with Copy rather than replaced by fresh variables.
  Errors
    `domain_error(acyclic_term, Term)` if Term is cyclic.
  Compatibility
    Quintus, SICStus. Not in YAP version of this library
[det]max_var_number(+Term, +Start, -Max)
  True when Max is the max of Start and the highest numbered $VAR(N) term.
  author
    Vitor Santos Costa
  Compatibility
    YAP
[det]varnumbers_names(+Term, -Copy, -VariableNames)
  If Term is a term with numbered and named variables using the reserved term '$VAR'(X), Copy is a copy of Term where each '$VAR'(X) is consistently replaced by a fresh variable and Bindings is a list `X = Var`, relating the X terms with the variable it is mapped to.
  See also
    numbervars/3, varnumbers/3, read_term/3 using the `variable_names` option.