public void processStartTraverser(final Traverser.Admin traverser) {
            if (null != this.matchKey)
                this.matchValue = this.getSafeScopeValue(Pop.last, this.matchKey, traverser);
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/filter/WhereTraversalStep.java