∀ {α : Type u_1} {n : Type u_3} {m : Type u_4} {A : Matrix m m α} {B : Matrix m n α} {C : Matrix n m α}
  {D : Matrix n n α}, (Matrix.fromBlocks A B C D).IsSymm ↔ A.IsSymm ∧ B.transpose = C ∧ C.transpose = B ∧ D.IsSymm