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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_node(self, process_name, timeperiod):
 """ Method retrieves a tree node identified by the time_qualifier and the timeperiod """

        if process_name not in self.process_hierarchy:
            raise ValueError('unable to retrieve the node due to unknown process: {0}'.format(process_name))

        time_qualifier = self.process_hierarchy[process_name].process_entry.time_qualifier
        return self._get_node(time_qualifier, timeperiod)