strange behavior of subset

Hello,

I used swi prolog Multi-threaded, 64 bits, Version 7.1.4
under Ubuntu.

I am suprise by the behavior of the subset predicate :


?- subset([p(A), q(A,B)], [p(1), p(2), q(2,3)]).
false.


?- subset([p(A), q(A,B)], [p(2), p(1), q(2,3)]).
A = 2,
B = 3.

Thus subset takes into account the order of the element
in the sets, what is not expected when one deals with sets.
  Is is normal ?

Thanks a lot

Dominique