void IndexSet::insert(const IndexSet& s) {
if (this == &s) return;
if (s.card() == 0) return;
if (card() == 0) {
*this = s;
return;
}
for (long i = s.last(); i >= s.first(); i = s.prev(i)) insert(i);
// NOTE: traversal done from high to low so as to trigger at
// at most one resize
}
