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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 _initialize_default_action_fluents(self):
        '''Returns the default action-fluents instantiated.'''
        action_fluents = self.rddl.domain.action_fluents
        self.default_action_fluents = self._initialize_pvariables(
            action_fluents,
            self.rddl.domain.action_fluent_ordering)
        return self.default_action_fluents