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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_class_repo(self, namespace):
 """ Returns the class 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 class repository does not contain the namespace yet, it is added. Parameters: namespace(:term:`string`):
 Namespace name. Must not be `None`. Returns: dict of CIMClass: Class repository. Raises: :exc:`~pywbem.CIMError`: CIM_ERR_INVALID_NAMESPACE: Namespace does not exist. """

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