gadt21.hs:21:60:
    Could not deduce (Ord a1) arising from a use of `f'
    from the context (a ~ Set a1)
      bound by a pattern with constructor
                 TypeSet :: forall a. Type a -> Type (Set a),
               in an equation for `withOrdDynExpr'
      at gadt21.hs:21:35-43
    Possible fix:
      add (Ord a1) to the context of
        the data constructor `TypeSet'
        or the data constructor `DynExpr'
        or the type signature for
             withOrdDynExpr :: DynExpr
                               -> (forall a. Ord a => Expr a -> b)
                               -> Maybe b
    In the first argument of `Just', namely `(f e)'
    In the expression: Just (f e)
    In an equation for `withOrdDynExpr':
        withOrdDynExpr (DynExpr e@(Const (TypeSet _) _)) f = Just (f e)