Actually, I'll check in a PairListRestraint in a sec when I can get
swig to behave.
If you want to do that, I propose subsequently renaming ListRestraint to
SingletonListRestraint and ChainTripletRestraint to
TripletListRestraint, since it'll get confusing otherwise. (Probably
quicker for me to do this myself than to force you to write a patch for
it though.)