fact: Definition Zermelo_chain E F := let p := fun a => a -s1 (rep a) in [/\ sub F (\Po E), inc E F, (forall A, inc A F -> inc (p A) F) & (forall A, sub A F -> nonempty A -> inc (intersection A) F)].
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)