return null;
return new UndoInsertString(where, strLen);
  public UndoableEdit insertString(int where, String str)      throws BadLocationException  {    // check arguments    int length = length();    int strLen = str.length();    if (where >= length)      throw new BadLocationException("the where argument cannot be greater"          + " than the content length", where);    replace(where, 0, str.toCharArray(), str.length());    return null;  }