proposition: Lemma scaleNmx : forall x A, (- x) *m: A === -m (x *m: A).
proof: Proof. by rewrite/null_mx/addmx/oppmx/scalemx=> x A i j; ring. Qed.
imports: ssreflect ssrbool ssrfun eqtype ssrnat seq choice fintype finfun bigops ssralg groups perm zmodp morphisms Ring RingClass bigopsClass Setoid Morphisms