code: function() {
      this.commit();
      if (this.redoHistory.length) {
        // The inverse of undo, basically.
        var item = this.redoHistory.pop();
        this.addUndoLevel(this.updateTo(item, "applyChain"));
        this.notifyEnvironment();
        return this.chainNode(item);
      }
    }
docs: Redo the last undone change.