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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def protocol_str(protocol):
 """Convert internal API protocol to string."""

    if protocol == const.PROTOCOL_MRP:
        return 'MRP'
    if protocol == const.PROTOCOL_DMAP:
        return 'DMAP'
    if protocol == const.PROTOCOL_AIRPLAY:
        return 'AirPlay'
    return 'Unknown'