proposition: Lemma igraphX x y: inverse_graph (x \times y) = y \times x.
proof: Proof. set_extens t. by move /igraphP => [pt /setX_P]; aw; move => [_ pa pb];apply: setX_i. move/setX_P=> [pt pa pb]; apply /igraphP; fprops. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)