SWI-Prolog -- var

Availability:built-in

var_property(+Var, ?Property)

True when Property is a property of Var. These properties are available during goal- and term-expansion. Defined properties are below. Future versions are likely to provide more properties, such as whether the variable is referenced in the remainder of the term. See also goal_expansion/2.

fresh(Bool): Bool has the value true if the variable is guaranteed to be unbound at entry of the goal, otherwise its value is false. This implies that the variable first appears in this goal or a previous appearance was in a negation (\+/1) or a different branch of a disjunction.
singleton(Bool): Bool has the value true if the variable is a syntactic singleton in the term it appears in. Note that this tests that the variable appears exactly once in the term being expanded without making any claim on the syntax of the variable. Variables that appear only once in multiple branches are not singletons according to this property. Future implementations may improve on that.
name(Name): True when variable appears with the given name in the source.