SWI-Prolog -- term

Availability:built-in

[ISO]term_variables(+Term, -List)

Unify List with a list of variables, each sharing with a unique variable of Term.^{108This
predicate used to be called free_variables/2 . The name term_variables/2
is more widely used. The old predicate is still available from the
library library(backcomp).} The variables in List are ordered in order of appearance traversing Term depth-first and left-to-right. See also term_variables/3 and nonground/2. For example:

?- term_variables(a(X, b(Y, X), Z), L).
L = [X, Y, Z].