proposition: Lemma inl_equiv_Iv S S': inl_same_data S S' -> inl_equiv S = inl_equiv S'.
proof: Proof. move => [sa sb sc]. by rewrite /inl_equiv/inl_sum /inl_equiv_rel sa sb sc. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)