In my opinion, as first option, in line 68 it is clearer to write:

er = log_q( q2.cast<T> * q1.conjugate() ); // This is a minus operation in global frame

than to twist the code with unclear shortcuts.

Otherwise, we could define plus_left(), plus_right(), minus_left() and minus_right(), as we defined Jac_SO3_left() and Jac_SO3_right() in rotations.h. But I would go for the first option.

OK then let's use log_q( q2.cast<T>() * q1.conjugate() ) as temporary fix and define new functions in rotations.h if really needed in other places.

I'm on it. Will do the following:

• create plus_right(), plus_left(), minus_right(), minus_left()
• have plus() = plus_right(), and minus() = minus_right()
• leave diff() = minus() as only option by now

Done and pushed to master. You can merge from master.

so now you can use minus_left(q1, q2)