Iterate values on specific depth.
        depth has to be greater equal than 0.
        Usage reference see :meth:`DictTree.kv_depth()<DictTree.kv_depth>`

def v_depth(d, depth):
        """Iterate values on specific depth.
        depth has to be greater equal than 0.
        Usage reference see :meth:`DictTree.kv_depth()<DictTree.kv_depth>`
        """
        if depth == 0:
            yield d
        else:
            for node in DictTree.v(d):
                for node1 in DictTree.v_depth(node, depth-1):
                    yield node1