docs: def _initialize_non_fluents(self):
          '''Returns the non-fluents instantiated.'''
          non_fluents = self.rddl.domain.non_fluents
          initializer = self.rddl.non_fluents.init_non_fluent
          self.non_fluents = self._initialize_pvariables(
              non_fluents,
              self.rddl.domain.non_fluent_ordering,
              initializer)
          return self.non_fluents