ppr var = ppr (varName var) <+> ifPprDebug (brackets (ppr_debug var))
ppr_debug :: Var -> SDoc
-ppr_debug (TyVar {}) = ptext (sLit "tv")
+ppr_debug (TyVar { isCoercionVar = False }) = ptext (sLit "tv")
+ppr_debug (TyVar { isCoercionVar = True }) = ptext (sLit "co")
ppr_debug (TcTyVar {tc_tv_details = d}) = pprTcTyVarDetails d
ppr_debug (Id { idScope = s, id_details = d }) = ppr_id_scope s <> pprIdDetails d