∀ {G : Type u_1} [inst : AddGroup G] {α : Type u_2} [inst_1 : AddAction G α] {H : AddSubgroup G} [hH : H.Normal] (g : G)
  {a : α}, a ∈ AddAction.fixedPoints (↥H) α → g +ᵥ a ∈ AddAction.fixedPoints (↥H) α