proposition: Lemma segment_rpC' r x : order r -> sub x (substrate r) -> (segment_rp r x <-> segmentp r (substrate r -s x)).
proof: Proof. move => or xsr; apply:iff_sym. rewrite - {2}(setC_K xsr). apply:(segment_rpC or); apply:(sub_setC). Qed.
imports: mathcomp(ssreflect ssrbool eqtype ssrfun)