Do I need to do all the pairs (AB, AC, AD, BC, BD, CD) or only the
minimal (AB, BC, CD)? In other words is the symmetry like the transitive
property of equality? Would C3 symmetry between chains ABC be (AB, AC,
BC) or just (AB, BC)?
It is a restraint, so if you restrain A to be like B and A to be like C
you do not need to restrain B to be like C.