proposition: Lemma tuto_subn_subA : forall m n p, p <= n -> m - (n - p) = m + p - n.
proof: Proof. move=> m n p le_pn. Search _ (_ + _ - _ = _) in ssrnat. Search _ (_ - _ + _). rewrite -{2}(subnK le_pn) // subnDr. done. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat div seq) mathcomp(path choice fintype tuple finset)