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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 DeregisterDefinition(self, artifact_definition):
 """Deregisters an artifact definition. Artifact definitions are identified based on their lower case name. Args: artifact_definition (ArtifactDefinition):
 an artifact definition. Raises: KeyError: if an artifact definition is not set for the corresponding name. """

    artifact_definition_name = artifact_definition.name.lower()
    if artifact_definition_name not in self._artifact_definitions:
      raise KeyError(
          'Artifact definition not set for name: {0:s}.'.format(
              artifact_definition.name))

    del self._artifact_definitions[artifact_definition_name]