code: function( event ) {
  			var nativeEvent = event.data.$;
  
  			if ( !( nativeEvent.ctrlKey || nativeEvent.metaKey ) ) {
  				this.remove( 17 );
  			}
  			if ( !nativeEvent.shiftKey ) {
  				this.remove( 16 );
  			}
  			if ( !nativeEvent.altKey ) {
  				this.remove( 18 );
  			}
  		}
docs: Cleans the stack based on a provided `keydown` event object. The rationale behind this method
  is that some keystrokes cause the `keydown` event to be fired in the editor, but not the `keyup` event.
  For instance, *Alt+Tab* will fire `keydown`, but since the editor is blurred by it, then there is
  no `keyup`, so the keystroke is not removed from the stack.
  
  @param {CKEDITOR.dom.event} event