def read_cBpack(filename):
    with gzip.open(filename, 'rb') as infile:
        data = msgpack.load(infile, raw=False)
    header = data[0]
    if (
        not isinstance(header, dict) or header.get('format') != 'cB'
        or header.get('version') != 1
    ):
        raise ValueError("Unexpected header: %r" % header)
    return data[1:]