proposition: Lemma Eq_setX2_S a b: (a \times b) \Eq (b \times a).
proof: Proof. have pa: sgraph (a \times b) by fprops. by move: (inv_graph_canon_fb pa); rewrite igraphX => / equipotent_aux. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)