ets correspond to mathematical sets
(i.e., the pointers are unique). Some sets are sorted to
enforce uniqueness. Some sets are ordered. For example,
the order of vertices in a ridge determine the ridge's
orientation. If you reverse the order of adjacent
vertices, the orientation reverses. Some sets are not
mathematical sets. They may be indexed as an array and
they may include NULL pointers. </p>
<p>The most common operation on a set is to iterate its
members. This is done with a 'FOREACH...' macro. Each se