∀ (R : Type u_1) (M₂ : Type u_3) (M₃ : Type u_4) [inst : Semiring R] [inst_1 : AddCommMonoid M₂] [inst_2 : Module R M₂]
  [inst_3 : AddCommMonoid M₃] [inst_4 : Module R M₃] {ι₁ : Type u_2} {ι₂ : Type u_5} (σ : ι₁ ≃ ι₂)
  (x y : MultilinearMap R (fun x => M₂) M₃),
  (MultilinearMap.domDomCongrEquiv σ).toFun (x + y) =
    (MultilinearMap.domDomCongrEquiv σ).toFun x + (MultilinearMap.domDomCongrEquiv σ).toFun y