numbervars/3

Availability:built-in

numbervars(+Term, +Start, -End)

Unify the free variables in Term with a term $VAR(N), where N is the number of the variable. Counting starts at Start. End is unified with the number that should be given to the next variable.^{bugOnly tagged
integers are supported (see the Prolog flag max_tagged_integer).
This suffices to count all variables that can appear in the largest term
that can be represented, but does not support arbitrary large integer
values for Start. On overflow, a representation_error(tagged_integer)
exception is raised.} The example below illustrates this. Note that the toplevel prints '$VAR'(0) as A due to the numbervars(true) option used to print answers.

?- Term = f(X,Y,X),
   numbervars(Term, 0, End, [singleton(true)]),
   write_canonical(Term), nl.
f('$VAR'(0),'$VAR'('_'),'$VAR'(0))
Term = f(A, _, A),
X = A,
Y = B,
End = 2.

See also the numbervars option to write_term/3 and numbervars/4.