/// <summary>
/// 这个函数被用来注册到文档的LengthChange事件，
/// 使文本被修改时，书签位置能够同步更新
/// </summary>
/// <param name="R">被修改的范围</param>
/// <param name="Change">修改前和修改后的片段的字数差异</param>
private void LengthChange(Range R, int Change)
{
var Begin = R.Start.Value;
var End = R.End.Value - 1;
switch (IInterval.CheckInInterval(Pos, Begin, End))
{
case IntervalPosition.Overflow:
Pos += Change;
break;
case IntervalPosition.Located when Change < 0:
Pos = Math.Min(Pos, End + Change);
break;
}
}