single property
Checks that this iterable has only one element, and returns that element.
Throws a StateError if this
is empty or has more than one element.
This operation will not iterate past the second element.
Implementation
E get single {
if (_head == _tail) throw IterableElementError.noElement();
if (length > 1) throw IterableElementError.tooMany();
return _table[_head] as E;
}