yoohoo081 Wrote:I don't get why X cannot be 1st in the diagram. I read that U cannot be later than X. But, why can't X be in 1st??
Notice that U has to come after T.
We also know that U is not in a later one than X.
So the X can go after the U or with the U but not before it!
This is like a chain
T
----
U with X or.... X comes after U.
No way X can be first.