<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def available_subtypes(format=None):
 """Return a dictionary of available subtypes. Parameters format : str If given, only compatible subtypes are returned. Examples -------- {'PCM_24': 'Signed 24 bit PCM', 'PCM_16': 'Signed 16 bit PCM', 'PCM_S8': 'Signed 8 bit PCM'} """

    subtypes = _available_formats_helper(_snd.SFC_GET_FORMAT_SUBTYPE_COUNT,
                                         _snd.SFC_GET_FORMAT_SUBTYPE)
    return dict((subtype, name) for subtype, name in subtypes
                if format is None or check_format(format, subtype))