proposition: Lemma mapf_disj s1 s2 s1' s2' : mapf s1 = mapf s2 -> mapf s1' = mapf s2' -> disj s1 s1' = disj s2 s2'.
proof: Proof. move=>X1 X2; apply: eq_trans (mapf_disjL _ X1) _. by rewrite !(disjC s2); apply: mapf_disjL X2. Qed.
imports: Coq(ssreflect ssrbool ssrfun) mathcomp(ssrnat eqtype seq path) pcm(options)