@Override
    public void remove() {
        if (this.current == 'b')
            this.a = null;
        else if (this.current == 'x')
            this.b = null;
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/util/iterator/DoubleIterator.java