∀ {M : Type u_3} {N : Type u_4} [inst : Mul M] [inst_1 : Mul N],
  MulHom.op =
    { toFun := fun f => { toFun := MulOpposite.op ∘ ⇑f ∘ MulOpposite.unop, map_mul' := ⋯ },
      invFun := fun f => { toFun := MulOpposite.unop ∘ ⇑f ∘ MulOpposite.op, map_mul' := ⋯ }, left_inv := ⋯,
      right_inv := ⋯ }