code: function() {
      // Make sure pending changes have been committed.
      this.commit();
  
      if (this.history.length) {
        // Take the top diff from the history, apply it, and store its
        // shadow in the redo history.
        var item = this.history.pop();
        this.redoHistory.push(this.updateTo(item, "applyChain"));
        this.notifyEnvironment();
        return this.chainNode(item);
      }
    }
docs: Undo the last change.