public void addTree(final Tree<T> tree) {
        tree.forEach((k, t) -> {
            if (this.containsKey(k)) {
                this.get(k).addTree(t);
            } else {
                this.put(k, t);
            }
        });
    }
@author Marko A. Rodriguez (http://markorodriguez.com)
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/util/Tree.java