+ rep_tc = famInstTyCon fam_inst
+ subst_domain = varEnvElts . getTvSubstEnv $ subst
+ tvs = map (Type.getTyVar "tcLookupFamInst")
+ subst_domain
+ variable_only_subst = all Type.isTyVarTy subst_domain &&
+ sizeVarSet (mkVarSet tvs) == length tvs
+ -- renaming may have no repetitions