private static <T> Map.Entry<T, Tree<T>> createTree(T key, Tree<T> tree) {
        return new AbstractMap.SimpleEntry<>(key, tree);
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
@author Daniel Kuppitz (http://gremlin.guru)
gremlin-core/src/test/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/TreeTest.java