@Override
    public void removeLocalChild(final Traversal.Admin<?, ?> traversal) {
        if (this.selectTraversal == traversal)
            this.selectTraversal = null;
        if (this.keyTraversal == traversal)
            this.keyTraversal = null;
    }
@author Daniel Kuppitz (http://gremlin.guru)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/TraversalSelectStep.java