SWI-Prolog -- var

Documentation
- Reference manual
  - Built-in Predicates
    - Analysing and Constructing Terms
- Packages

Availability:built-in

var_number(@Term, -VarNumber)

True if Term is numbered by numbervars/3 and VarNumber is the number given to this variable. This predicate avoids the need for unification with '$VAR'(X) and opens the path for replacing this valid Prolog term by an internal representation that has no textual equivalent.