Section MatrixAlgebra.
Variables F : fieldType.
Local Notation "A \in R" := (@submx F _ _ _ (mxvec A) R).
Arguments memmx_subP {m1 m2 n R1 R2}.
Arguments memmx_eqP {m1 m2 n R1 R2}.
Arguments memmx_addsP {m1 m2 n A R1 R2}.
Arguments memmx_sumsP {I P n A R_}.

Lemma has_non_scalar_mxP m n (R : 'A_(m, n)) :
(1%:M \in R)%MS ->
reflect (exists2 A, A \in R & ~~ is_scalar_mx A)%MS (1 < \rank R).