variant_term/2 is faster than variant/1 (Was: Request for comments, Novacore the sequel to ISO modules)

The ISO core standard probably set the
stage for a couple of performance sins.
In 7.1.6.1 Variants of a term we find
these test cases:
- f(A, B, A) is a variant of f(X, Y, X).
- g(A, B) is a variant of g(_, _).
- P+Q is a variant of P+Q.
What is doubious here, is the last test
case with P+Q. Do we need to test terms
that have common variables?
Lets assume we have situations where we
don't need variant working with common
variables in the two argument terms, what
about then using this bootstrapping:
variant_term(X, Y) :-
    subsumes_term(X, Y),
    subsumes_term(Y, X).
Here some testing, does it work ok? Take this code:
enum_arg(_, 1).
enum_arg(_, _).
enum_arg(X, X).
enum_list(_, []).
enum_list(X, [H|T]) :- enum_arg(X, H), enum_list(X, T).
boole(G, 1) :- G, !.
boole(_, 0).
nok(L, R) :- length(L, 6), length(R, 6),
      enum_list(_, L), enum_list(_, R),
      boole(variant(L, R), A), boole(variant_term(L, R), B),
      A \== B.
Seems to work fine:
?- nok(L, R).
false.