You will now complete your own first proof about subrelations.
"R ⊆ S" means that R is a subrelation of S. That means if R[x,y] holds, then also S[x,y].
The lemma proves that if R is a subrelation of S and S is a subrelation of T, then R must be a subrelation of T as well.
Try to complete the proof by inserting something for "{INSERT-SOLUTION-HERE}"!
Note that the proof markers "Assume", "Then" and "Hence" have different meanings: