@Override
        public Tree apply(final Tree mutatingSeed, final Tree tree) {
            mutatingSeed.addTree(tree);
            return mutatingSeed;
        }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/TreeStep.java