@Override
    public Traverser.Admin<E> nextTraverser() {
        if (null != this.bypassTraversal)
            return this.bypassTraversal.nextTraverser();
        throw new UnsupportedOperationException("The " + this.getClass().getSimpleName() + " can only be used as a predicate traversal");
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/lambda/AbstractLambdaTraversal.java