(* * Maximum element in a list *)
From Undecidability Require Export TM.Util.Prelim TM.Util.ArithPrelim.
(* ** Basic lemmas about upper bounds *)
(* ** Tail Recursive Definition *)
(* Compute the maximum of a list and a start-value tail-recursively *)
Fixpoint max_list_rec (s : nat) (xs : list nat) { struct xs } : nat :=
match xs with
| nil => s
| x :: xs' => max_list_rec (max x s) xs'
end.

(* ... and also [s] is equal to [max_list_rec s xs]. *)
Corollary max_list_rec_gt' xs s :
(forall x : nat, x el xs -> x < max_list_rec s xs) ->
max_list_rec s xs = s /\ (forall x : nat, x el xs -> x < s).