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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 _validate_namespace(self, namespace):
 """ Validate whether a CIM namespace exists in the mock repository. Parameters: namespace (:term:`string`):
 The name of the CIM namespace in the mock repository. Must not be `None`. Raises: :exc:`~pywbem.CIMError`: CIM_ERR_INVALID_NAMESPACE: Namespace does not exist. """

        if namespace not in self.namespaces:
            raise CIMError(
                CIM_ERR_INVALID_NAMESPACE,
                _format("Namespace does not exist in mock repository: {0!A}",
                        namespace))