Section ZL.
(* get rid of the need for a sup function and immediate successors *)
Variable T:Type.
Variable R:relation T.
Hypothesis ord: order R.
Hypothesis ub_of_chain: forall S:Ensemble T, chain R S ->
exists x:T, forall y:T, In S y -> R y x.
Definition chains := {S:Ensemble T | chain R S}.
Definition chains_ord := (fun S1 S2:chains =>
Included (proj1_sig S1) (proj1_sig S2)).
Definition chains_sup_def : forall F: Ensemble chains,
chain chains_ord F -> chains.
refine (fun F H => exist _ [ x:T | exists S:chains, In F S /\
In (proj1_sig S) x ] _).
red; intros.
destruct H0, H1, H0, H1.
pose proof (H x0 x1).
destruct x0, x1, H0, H1.
apply H2 in H0; trivial.
destruct H0;
apply c0 + apply c;
trivial;
now apply H0.
Defined.
Definition chains_sup (F:Ensemble chains) (P:chain chains_ord F) :=
let U := chains_sup_def F P in
exist (fun U:chains =>
(forall S:chains, In F S -> chains_ord S U) /\
(forall T:chains, (forall S:chains, In F S -> chains_ord S T) ->
chains_ord U T))
(chains_sup_def F P) (chains_sup_correct F P).

Theorem ZornsLemma: exists x:T, maximal R x.