code: @Override
    public void setUndoableEditListener( UndoableEditListener uel )
    {
      if( _uel != null )
      {
        getEditor().getDocument().removeUndoableEditListener( _uel );
      }
  
      _uel = uel;
  
      if( _uel != null )
      {
        getEditor().getDocument().addUndoableEditListener( _uel );
      }
    }
docs: Sets the one and only undoable edit listener for this editor section.
  The primary use case for this method is to establish an undo manager
  connection.
  
  @param uel The UndoableEditListener to connect to this section's document.