resetMarksAtZero();
  protected void shiftGapEndUp(int newGapEnd)  {    if (newGapEnd == gapEnd)      return;    assert newGapEnd > gapEnd : "The new gap end must be greater than the "                                + "old gap end.";    setPositionsInRange(gapEnd, newGapEnd, false);    gapEnd = newGapEnd;  }