TrJoin<TR1, TR2, ...>
A little hard to explain, but all the specified transitions are run in parallel. Basically, we chain transitions like ((A TR1 B) TR2 B)
TrJoinR
https://github.com/profezzorn/ProffieOS/blob/6f8add544c627172ad2dd698c90e5e55078a420a/transitions/join.h#L4