def testA():

    # test A
    assert packet.pack('A','C') == b'AC'
    assert packet.pack('A','B') == b'AB'
    assert packet.pack('A','C') != b'AB'

    assert packet.unpack_stream(io.BytesIO(b'AC')) == ('A', 'C')
    assert packet.unpack_stream(io.BytesIO(b'AB')) == ('A', 'B')
    assert packet.unpack_stream(io.BytesIO(b'AB')) != ('A', 'C')