induction n generalizing x y
C : Container
R : M C → M C → Prop
h₀ :
  ∀ (x y : M C),
    R x y →
      ∃ node k₁ k₂,
        destruct x = { fst := node, snd := k₁ } ∧
          destruct y = { fst := node, snd := k₂ } ∧ ∀ (i : B C node), R (k₁ i) (k₂ i)
x y : M C
n : Nat
h₁ : R x y
⊢ approx x n = approx y n
case zero
C : Container
R : M C → M C → Prop
h₀ :
  ∀ (x y : M C),
    R x y →
      ∃ node k₁ k₂,
        destruct x = { fst := node, snd := k₁ } ∧
          destruct y = { fst := node, snd := k₂ } ∧ ∀ (i : B C node), R (k₁ i) (k₂ i)
x y : M C
h₁ : R x y
⊢ approx x Nat.zero = approx y Nat.zero

case succ
C : Container
R : M C → M C → Prop
h₀ :
  ∀ (x y : M C),
    R x y →
      ∃ node k₁ k₂,
        destruct x = { fst := node, snd := k₁ } ∧
          destruct y = { fst := node, snd := k₂ } ∧ ∀ (i : B C node), R (k₁ i) (k₂ i)
n✝ : Nat
n_ih✝ : ∀ (x y : M C), R x y → approx x n✝ = approx y n✝
x y : M C
h₁ : R x y
⊢ approx x (Nat.succ n✝) = approx y (Nat.succ n✝)