We have {ke} or {tu'e} for any grouping (la jbofi'e has limitation in {ke}, and la camxes and la ilmentufa inherited it, but it is resolved in la zantufa_0.16 or later).
Besides right-grouping, {bo} has un-substitutable role in the grammar as short scope joiner, and we have no short scope joiner in left-grouping (similarly, we have no long scope joiner in right-grouping other than {co}). This lack brings problems in practical use.
In order to make the grammar more useful, we would have two alternatives.
1. all joiners do grouping to the same direction, and use {ke} or {tu'e} for different grouping.
2. create two cmavo for each scope of joiner, one for left-grouping, and another for right-grouping.
I prefer 1 for simplicity, and did it in my parser la zantufa.