Add a unit test
Add a unit test

The unit test of `PreqOrder()` has been added.

jojohappy/falcon-plus,jojohappy/falcon-plus,open-falcon/falcon-plus,taomaree/falcon-plus,taomaree/falcon-plus,open-falcon/falcon-plus,taomaree/falcon-plus,open-falcon/falcon-plus,taomaree/falcon-plus,jojohappy/falcon-plus,jojohappy/falcon-plus,open-falcon/falcon-plus,jojohappy/falcon-plus,taomaree/falcon-plus,jojohappy/falcon-plus,taomaree/falcon-plus,open-falcon/falcon-plus