fact: Definition worder_of E := let om := intersection (Zo (\Po (\Po E)) (Zermelo_chain E)) in let d:= fun x => intersection (Zo om (sub x)) in let R := fun x => d (singleton x) in graph_on (fun x y => (sub (R y) (R x))) E.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)