proposition: Lemma order_graph_r_P x r z z': order_graph_r x r z z' <-> order_product_r (cst_graph x r) z z'.
proof: Proof. have dc: (domain (cst_graph x r) = x) by aw. rewrite /order_product_r dc /cst_graph. split; first by move=> go i ix; rewrite LgV //; apply: go. move=> h y yx; move: (h _ yx); rewrite LgV //. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)