@Override public Option<E> head() {
        return size() > 0 ? Option.some(impl.firstKey()) : Option.none();
    }
@Override public Option<E> head() {
        return size() > 0 ? Option.some(impl.firstKey()) : Option.none();
    }
@ Override public Option < E > head ( ) { return size ( ) > 0 ? Option . some ( impl . firstKey ( ) ) : Option . none ( ) ; }
{@inheritDoc}
{