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}.

Lemma memmx_sumsP (I : finType) (P : pred I) n (A : 'M_n) R_ :
reflect (exists2 A_, A = \sum_(i | P i) A_ i & forall i, A_ i \in R_ i)
(A \in \sum_(i | P i) R_ i)%MS.