-- Lean 4 proof step from Mathlib.Topology.Metrizable.Uniformity (goal)
Goal:
X : Type u_1
d : X → X → ℝ≥0
dist_self : ∀ (x : X), d x x = 0
dist_comm : ∀ (x y : X), d x y = d y x
hd : ∀ (x₁ x₂ x₃ x₄ : X), d x₁ x₄ ≤ 2 * max (d x₁ x₂) (max (d x₂ x₃) (d x₃ x₄))
x y : X
l : List X
this : IsTrans X fun (x y : X) ↦ d x y = 0
n : ℕ
hn : l.length = n
⊢ d x y ≤ 2 * (zipWith d (x :: l) (l ++ [y])).sum
have h : ∀ (n : ℕ) (ihn : ∀ m < n, ∀ (x y : X) (l : List X) (hn : l.length = m), d x y ≤ 2 * (zipWith d (x :: l) (l ++ [y])).sum)
  (x y : X) (l : List X) (hn : l.length = n), d x y ≤ 2 * (zipWith d (x :: l) (l ++ [y])).sum
Tactic: induction n using Nat.strong_induction_on generalizing x y l with | h n ihn =>
  simp only at ihn
  subst n
  set L := zipWith d (x::l) (l ++ [y])
  have hL_len : length L = length l + 1 := by simp [L]
  rcases eq_or_ne (d x y) 0 with hd₀ | hd₀
  · simp only [hd₀, zero_le]
  rsuffices ⟨z, z', hxz, hzz', hz'y⟩ : ∃ z z' : X, d x z ≤ L.sum ∧ d z z' ≤ L.sum ∧ d z' y ≤ L.sum
  · grw [hd x z z' y, max_le hxz (max_le hzz' hz'y)]
  set s : Set ℕ := { m : ℕ | 2 * (take m L).sum ≤ L.sum }
  have hs₀ : 0 ∈ s := by simp [s]
  have hsne : s.Nonempty := ⟨0, hs₀⟩
  obtain ⟨M, hMl, hMs⟩ : ∃ M ≤ length l, IsGreatest s M := by
    have hs_ub : length l ∈ upperBounds s := by
      intro m hm
      rw [← not_lt, Nat.lt_iff_add_one_le, ← hL_len]
      intro hLm
      rw [mem_setOf_eq, take_of_length_le hLm, two_mul, add_le_iff_nonpos_left, nonpos_iff_eq_zero,
          sum_eq_zero_iff, ← forall_iff_forall_mem, forall_zipWith,
          ← isChain_cons_append_singleton_iff_forall₂]
          at hm <;>
        [skip; simp]
      exact hd₀ (hm.rel_cons (mem_append.2 <| Or.inr <| mem_singleton_self _))
    have hs_bdd : BddAbove s := ⟨length l, hs_ub⟩
    exact ⟨sSup s, csSup_le hsne hs_ub, ⟨Nat.sSup_mem hsne hs_bdd, fun k => le_csSup hs_bdd⟩⟩
  have hM_lt : M < length L := by rwa [hL_len, Nat.lt_succ_iff]
  have hM_ltx : M < length (x::l) := lt_length_left_of_zipWith hM_lt
  have hM_lty : M < length (l ++ [y]) := lt_length_right_of_zipWith hM_lt
  refine ⟨(x::l)[M], (l ++ [y])[M], ?_, ?_, ?_⟩
  · cases M with
    | zero =>
      simp [dist_self]
    | succ M =>
      rw [Nat.succ_le_iff] at hMl
      have hMl' : length (take M l) = M := length_take.trans (min_eq_left hMl.le)
      refine (ihn _ hMl _ _ _ hMl').trans ?_
      convert! hMs.1.out
      rw [take_zipWith, take, take_add_one, getElem?_append_left hMl, getElem?_eq_getElem hMl,
        ← Option.coe_def, Option.toList_some, take_append_of_le_length hMl.le, getElem_cons_succ]
  · exact single_le_sum (fun x _ => zero_le) _ (mem_iff_get.2 ⟨⟨M, hM_lt⟩, getElem_zipWith⟩)
  · rcases hMl.eq_or_lt with (rfl | hMl)
    · simp only [getElem_append_right le_rfl, getElem_singleton, dist_self, zero_le]
    rw [getElem_append_left hMl]
    have hlen : length (drop (M + 1) l) = length l - (M + 1) := length_drop
    have hlen_lt : length l - (M + 1) < length l := Nat.sub_lt_of_pos_le M.succ_pos hMl
    refine (ihn _ hlen_lt _ y _ hlen).trans ?_
    rw [cons_getElem_drop_succ]
    have hMs' : L.sum ≤ 2 * (L.take (M + 1)).sum :=
      not_lt.1 fun h => (hMs.2 h.le).not_gt M.lt_succ_self
    rw [← sum_take_add_sum_drop L (M + 1), two_mul, add_le_add_iff_left, ← add_le_add_iff_right,
      sum_take_add_sum_drop, ← two_mul] at hMs'
    convert! hMs'
    rwa [drop_zipWith, drop, drop_append_of_le_length]
Cleared hypotheses: x, y, l, n, hn