unique rectangle type 1~4没什么疑问，不过hidden unique rectangle 的删除有些疑惑，如何确定哪个是候选数a，哪个是候选数b的呢
有些技巧似乎国内的那个论坛上也并没有提到，国外那位大神总结的真是全。
Generalising X-Wing
X-Wing is not restricted to rows and columns. We can also extend the idea to boxes as well. If we generalise the rule above we get:
When there are * only 2 candidates for a value, in each of 2 different units of the same kind, * and these candidates lie also on 2 other units of the same kind,
then all other candidates for that value can be eliminated from the latter two units.
Now we have 6 combinations:
- Starting from 2 rows and eliminating in 2 columns -> Classic X-Wing
- Starting from 2 columns and eliminating in 2 rows -> Classic X-Wing
- Starting from 2 boxes and eliminating in 2 rows -> Same effect as line/box reduction
- Starting from 2 boxes and eliminating in 2 columns -> Same effect as line/box reduction
- Starting from 2 rows and eliminating in 2 boxes -> Same effect as pointing pairs
- Starting from 2 columns and eliminating in 2 boxes -> Same effect as pointing pairs