description

# single property Null safety

E 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.

## Implementation

    E get single {
      // Note that this throws correctly if the queue is empty
      // because reading the element of the sentinel throws.
      if (identical(_sentinel._nextLink, _sentinel._previousLink)) {
        DoubleLinkedQueueEntry<E> entry = _sentinel._nextLink!;
        return entry._element as E;
      }
      throw IterableElementError.tooMany();
    }