For two disjoint sets A and B we have two possible
situations: they can be complementary or mutually exclusive sets.
If we denote by A'_{C} the complement of
A with regard to some larger set C containing A
(i.e. A'_{C}=C \ A, where A is a subset of C),
it is also possible to take for C the universal set,
so the complement of A is "no A". In this case we have the
classical relation "figure""ground".
