proper import of Main: BNF_Least_Fixpoint does not "contain pretty much everything", especially it lacks the 'value' command, which is defined *after* theory List;
theory ToyList
imports Main
begin
no_notation Nil ("[]") and Cons (infixr "#" 65) and append (infixr "@" 65)
hide_type list
hide_const rev
datatype 'a list = Nil ("[]")
| Cons 'a "'a list" (infixr "#" 65)
(* This is the append function: *)
primrec app :: "'a list => 'a list => 'a list" (infixr "@" 65)
where
"[] @ ys = ys" |
"(x # xs) @ ys = x # (xs @ ys)"
primrec rev :: "'a list => 'a list" where
"rev [] = []" |
"rev (x # xs) = (rev xs) @ (x # [])"