Arguments sub_addsmxP {m1 m2 m3 n A B C}.
Variable I : finType.
Implicit Type P : pred I.

Lemma genmx_sums P n (B_ : I -> 'M_n) :
<<(\sum_(i | P i) B_ i)%MS>>%MS = (\sum_(i | P i) <<B_ i>>)%MS.