proposition: Lemma Exercise4_8f K: sub K (substrate r) -> order_on (opp_order (induced_order r K)) K.
proof: Proof. move: Exercise4_7b => or Ksr. move: (iorder_osr or Ksr) => [or1 {2} <-]; exact: (opp_osr or1). Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)