add method
Adds the element e
to this, and returns wether the element was
succesfully added or not.
You can always add elements, even duplicated elemneted are added, so this always return true.
Implementation
bool add(E e) {
_length++;
final added = _backingSet.add([e]);
if (!added) {
_backingSet.lookup([e])!.add(e);
}
return true;
}