This means that every row in the point table must have a key value (consisting, in this case, of two column values) that must also exist in the source table.
这意味着点表中每一行的键值(在这个示例中,由两个列值组成)必须也在源表中存在。
2
The token represents a relative point in the modification sequence of a row.
这个标记表示某一行的修改序列中的一个相对点。
3
Basically, the purpose of this indexer is to return a point corresponding to a given row and column on the drawing surface. Follow these steps.