Iterate keys at specified depth.

def keys_at(self, depth, counter=1):
        """
        Iterate keys at specified depth.
        """
        if depth < 1:
            yield ROOT
        else:
            if counter == depth:
                for key in self.keys():
                    yield key
            else:
                counter += 1
                for dict_tree in self.values():
                    for key in dict_tree.keys_at(depth, counter):
                        yield key