∀ {m : Type u_2} {n : Type u_3} {R : Type u_14} {α : Type u_15} {b : m → α} (M : Matrix m m (Matrix n n R)) (a : α),
  ((Matrix.comp m m n n R) M).toSquareBlock (fun i => b i.1) a =
    (Matrix.reindex Equiv.prodSubtypeFstEquivSubtypeProd.symm Equiv.prodSubtypeFstEquivSubtypeProd.symm)
      ((Matrix.comp { a_1 // b a_1 = a } { a_1 // b a_1 = a } n n R) (M.toSquareBlock b a))