1theory ToyList 2imports Main 3begin 4 5no_notation Nil ("[]") and Cons (infixr "#" 65) and append (infixr "@" 65) 6hide_type list 7hide_const rev 8 9datatype 'a list = Nil ("[]") 10 | Cons 'a "'a list" (infixr "#" 65) 11 12(* This is the append function: *) 13primrec app :: "'a list => 'a list => 'a list" (infixr "@" 65) 14where 15"[] @ ys = ys" | 16"(x # xs) @ ys = x # (xs @ ys)" 17 18primrec rev :: "'a list => 'a list" where 19"rev [] = []" | 20"rev (x # xs) = (rev xs) @ (x # [])" 21