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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 _get_instance_repo(self, namespace):
 """ Returns the instance repository for the specified CIM namespace within the mock repository. This is the original instance variable, so any modifications will change the mock repository. Validates that the namespace exists in the mock repository. If the instance repository does not contain the namespace yet, it is added. Parameters: namespace(:term:`string`):
 Namespace name. Must not be `None`. Returns: list of CIMInstance: Instance repository. Raises: :exc:`~pywbem.CIMError`: CIM_ERR_INVALID_NAMESPACE: Namespace does not exist. """

        self._validate_namespace(namespace)
        if namespace not in self.instances:
            self.instances[namespace] = []
        return self.instances[namespace]