+ famSubst = ASSERT( length (tyConTyVars tycon ) ==
+ length (mkTyVarTys univ_tvs) )
+ zipTopTvSubst (tyConTyVars tycon) (mkTyVarTys univ_tvs)
+ -- substitution mapping the type constructor's type
+ -- arguments to the universals of the data constructor
+ -- (crucial when type checking interfaces)