I think here it is important to note that each workshop is a two variable block. LL vs. PP vs. RR vs. SS.
The last rule is simply saying that LL comes before both PP and RR. LL has to be fully finished before PP and RR can start.
This game can be set up using two layers just for convenience sake. Because it is possible that more than one workshop occurs on a given day. So that way we can demonstrate an overlap between PP and RR.
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M T W TH F
I hope this helps. Please let me know if you have any other questions.