∀ {M : Type u_1} {α : Type u_2} [inst : AddGroup M] [inst_1 : AddAction M α] {s : Set α} {n : ℕ} [inst_2 : Finite ↑s]
  (x : Fin n ↪ ↥(SubAddAction.ofFixingAddSubgroup M s)),
  SubAddAction.ofFixingAddSubgroup.append x = Fin.Embedding.append ⋯