Arguments cent_mxP {m n B R}.

Lemma mxdirect_adds_center m1 m2 n (R1 : 'A_(m1, n)) (R2 : 'A_(m2, n)) :
mx_ideal (R1 + R2)%MS R1 -> mx_ideal (R1 + R2)%MS R2 ->
mxdirect (R1 + R2) ->
('Z((R1 + R2)%MS) :=: 'Z(R1) + 'Z(R2))%MS.