def _pc(piece_string: bytes, *, length: int = 20, start: int = 0):
    """
    Pieces a bytestring into pieces of specified length.

    :param piece_string: string to piece
    :param length:       piece length
    :return:             generator expression yielding pieces
    """
    return (piece_string[0 + i:length + i] for i in
            range(start, len(piece_string), length))