code: function construct_statepen(raw_params) {
          const statepen = this;
          const { store } = validate_params(statepen, raw_params);
          const creator_args = [ statepen, store ];
          statepen.get = create_getter(...creator_args);
          statepen.stream = create_streamer(...creator_args);
          statepen.mutate = create_mutator(...creator_args);
          return statepen;
      }
docs: -----------