numbervars/3 console output

Dear Jan,

http://www.swi-prolog.org/pldoc/doc_for?object=numbervars/3 says that

?- numbervars(foo(A, B, A), 0, End).
A = '$VAR'(0),
B = '$VAR'(1),
End = 2.

but when I run it I get (on any 6.6.x and 7.1.x version)

?- numbervars(foo(A, B, A), 0, End).
A = A,
B = B,
End = 2.

That's highly confusing, since it looks as if nothing happens.

Regards,
Günter