∀ (n : ℕ) (x : Fin (n + 3)),
  SimpleGraph.cycleGraph_EulerianCircuit_cons✝ n x =
    match x with
    | ⟨0, h⟩ => SimpleGraph.Walk.nil
    | ⟨m.succ, h⟩ =>
      have hadj := ⋯;
      SimpleGraph.Walk.cons hadj (SimpleGraph.cycleGraph_EulerianCircuit_cons✝¹ n ⟨m, ⋯⟩)