proposition: Lemma maximal_opp r: order r -> forall x, (maximal (opp_order r) x <-> minimal r x).
proof: Proof. move=> or x; rewrite /maximal /minimal (proj2 (opp_osr or)); split; move => [xsr h]; split=> // y /igraph_pP;apply: h. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)