add method
Adds the element e
to this, and returns whether the element was
added or not. If the element already exists in the collection, it isn't
added.
Implementation
@override
bool add(E e) {
final elementSet = {e};
var added = _backingSet.add(elementSet);
final isRootSet = added;
if (!isRootSet) {
added = _backingSet.lookup(elementSet)!.add(e);
}
if (added) {
_length++;
_validReverseCache = false;
}
return added;
}