∀ {m : Type u_1} {n : Type u_2} [inst : DecidableEq n] [inst_1 : Fintype n] [inst_2 : DecidableEq m]
  [inst_3 : Fintype m] {R : Type v} [inst_4 : CommRing R] (A : Matrix m m R) (C : Matrix n m R) (D : Matrix n n R),
  (Matrix.fromBlocks A 0 C D).det = A.det * D.det