= do { result@(rep_tycon, rep_tys) <- tcLookupFamInst tycon tys
; let { tvs = map (Type.getTyVar
"TcDeriv.tcLookupFamInstExact")
- tys
+ rep_tys
; variable_only_subst = all Type.isTyVarTy rep_tys &&
sizeVarSet (mkVarSet tvs) == length tvs
-- renaming may have no repetitions