description

# single property

E get single
override

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 (_count == 1) return _root!.key;
      throw _count == 0
          ? IterableElementError.noElement()
          : IterableElementError.tooMany();
    }