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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 add_metric_group_definition(self, definition):
 """ Add a faked metric group definition. The definition will be used: * For later addition of faked metrics responses. * For returning the metric-group-info objects in the response of the Create Metrics Context operations. For defined metric groups, see chapter "Metric groups" in the :term:`HMC API` book. Parameters: definition (:class:~zhmcclient.FakedMetricGroupDefinition`):
 Definition of the metric group. Raises: ValueError: A metric group definition with this name already exists. """

        assert isinstance(definition, FakedMetricGroupDefinition)
        group_name = definition.name
        if group_name in self._metric_group_defs:
            raise ValueError("A metric group definition with this name "
                             "already exists: {}".format(group_name))
        self._metric_group_defs[group_name] = definition
        self._metric_group_def_names.append(group_name)