docs: 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