@Override
        public int hashCode() {
            return super.hashCode() ^ (null == this.selectKey ? "null".hashCode() : this.selectKey.hashCode());
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/filter/WhereTraversalStep.java