assert(mark <= gapStart || mark > gapEnd);
assert mark <= gapStart || mark > gapEnd : "mark: " + mark + ", gapStart: " + gapStart + ", gapEnd: " + gapEnd;
    public int getOffset()    {      // Check precondition.      assert(mark <= gapStart || mark > gapEnd);      if (mark <= gapStart)        return mark;      else        return mark - (gapEnd - gapStart);    }