import itertools


def flatten(sequence_list, cls=list):
    """
    Flatten one level of nesting
    :param sequence_list: list of sequence
    :param cls: create instance of cls by flatten_gen
    :return: cls instance or generator
    """
    flatten_gen = itertools.chain.from_iterable(sequence_list)
    return cls(flatten_gen) if cls else flatten_gen