\begin{code}
derivingThingErr clas tys tycon tyvars why
= sep [hsep [ptext SLIT("Can't make a derived instance of"), quotes (ppr pred)],
- parens why]
+ nest 2 (parens why)]
where
pred = mkClassPred clas (tys ++ [mkTyConApp tycon (mkTyVarTys tyvars)])