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