∀ {m : Type u_2} {n : Type u_3} {R : Type u_14} [inst : Zero R] [inst_1 : DecidableEq m] (d : m → Matrix n n R),
  (Matrix.comp m m n n R) (Matrix.diagonal d) =
    (Matrix.reindex (Equiv.prodComm n m) (Equiv.prodComm n m)) (Matrix.blockDiagonal d)