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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 DeregisterSourceType(cls, source_type_class):
 """Deregisters a source type. Source types are identified based on their type indicator. Args: source_type_class (type):
 source type. Raises: KeyError: if a source type is not set for the corresponding type indicator. """

    if source_type_class.TYPE_INDICATOR not in cls._source_type_classes:
      raise KeyError(
          'Source type not set for type: {0:s}.'.format(
              source_type_class.TYPE_INDICATOR))

    del cls._source_type_classes[source_type_class.TYPE_INDICATOR]