@Override
    public T next() {
        if (this.current == 'x')
            throw FastNoSuchElementException.instance();
        else {
            if (this.current == 'a') {
                this.current = 'b';
                return this.a;
            } else {
                this.current = 'x';
                return this.b;
            }
        }
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/util/iterator/DoubleIterator.java