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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 RegisterDecompressor(cls, decompressor):
 """Registers a decompressor for a specific compression method. Args: decompressor (type):
 decompressor class. Raises: KeyError: if the corresponding decompressor is already set. """

    compression_method = decompressor.COMPRESSION_METHOD.lower()
    if compression_method in cls._decompressors:
      raise KeyError(
          'Decompressor for compression method: {0:s} already set.'.format(
              decompressor.COMPRESSION_METHOD))

    cls._decompressors[compression_method] = decompressor