docs: def _initialize_initial_state_fluents(self):
          '''Returns the initial state-fluents instantiated.'''
          state_fluents = self.rddl.domain.state_fluents
          initializer = self.rddl.instance.init_state
          self.initial_state_fluents = self._initialize_pvariables(
              state_fluents,
              self.rddl.domain.state_fluent_ordering,
              initializer)
          return self.initial_state_fluents