varnumbers/2 in B-Prolog

varnumbers.pl

% unnumber_vars/2 is not working correctly in B-Prolog 8.1.  Here is
% an alternative implementation of unnumber_vars/2 named varnumbers/2,
% which is compatible with SWI-Prolog's varnumbers library.

max_varnumber('$VAR'(N), Max) =>
    Max = N.
max_varnumber(Term, Max) =>
    Term =.. [_ | Args0],
    Args @= [Arg : Arg0 in Args0, [Arg], max_varnumber(Arg0, Arg)],
    Max is max([-1 | Args]).

% The number of variables in Term is limited to 65535.
varnumbers(Term, Copy) =>
    max_varnumber(Term, Max),
    Arity is Max + 1,
    functor(Vars, vars, Arity),
    varnumbers(Term, Vars, Copy).

varnumbers('$VAR'(N), Vars, Copy) =>
    I is N + 1,
    arg(I, Vars, Copy).
varnumbers(Term, Vars, Copy) =>
    Term =.. [Name | Args0],
    Args @= [Arg : Arg0 in Args0, [Arg], varnumbers(Arg0, Vars, Arg)],
    Copy =.. [Name | Args].