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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 value(source, key, ext=COMPLETE):
 """Extracts value for the specified metadata key from the given extension set. Keyword arguments: source -- string containing MultiMarkdown text ext -- extension bitfield for processing text key -- key to extract """

    _MMD_LIB.extract_metadata_value.restype = ctypes.c_char_p
    _MMD_LIB.extract_metadata_value.argtypes = [ctypes.c_char_p, ctypes.c_ulong, ctypes.c_char_p]

    src = source.encode('utf-8')
    dkey = key.encode('utf-8')

    value = _MMD_LIB.extract_metadata_value(src, ext, dkey)
    return value.decode('utf-8') if value else ''