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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_obs_function(self):
 """ add observation function tag to pomdpx model Return string containing the xml for observation function tag """

        obs_function = self.model['obs_function']
        for condition in obs_function:
            condprob = etree.SubElement(self.observation_function, 'CondProb')
            self.add_conditions(condition, condprob)
        return self.__str__(self.observation_function)[:-1]