-- Lean 4 proof step from Mathlib.Topology.EMetricSpace.BoundedVariation (goal)
Goal:
α : Type u_1
inst : LinearOrder α
E : Type u_2
inst_1 : PseudoEMetricSpace E
f : α → E
s : Set α
x : α
hx : x ∈ s
u : ℕ → α
hu : Monotone u
us : ∀ (i : ℕ), u i ∈ s
n : ℕ
h : x < u n
exists_N : ∃ N ≤ n, x < u N
N : ℕ := Nat.find (p := fun (n_1 : ℕ) ↦ n_1 ≤ n ∧ x < u n_1) exists_N
hN : N ≤ n ∧ x < u N
w : ℕ → α := fun (i : ℕ) ↦ if i < N then u i else if i = N then x else u (i - 1)
hw : Monotone w
Npos : 0 < N
i :
  ∑ i ∈ Finset.range n, edist (f (u (i + 1))) (f (u i)) =
    HAdd.hAdd (α := ℝ≥0∞) (∑ i ∈ Finset.Ico 0 (N - 1), edist (f (u (i + 1))) (f (u i)))
        (∑ i ∈ Finset.Ico (N - 1) N, edist (f (u (i + 1))) (f (u i))) +
      ∑ i ∈ Finset.Ico N n, edist (f (u (i + 1))) (f (u i))
i₁ :
  HAdd.hAdd (α := ℝ≥0∞) (∑ i ∈ Finset.Ico 0 (N - 1), edist (f (u (i + 1))) (f (u i)))
        (∑ i ∈ Finset.Ico (N - 1) N, edist (f (u (i + 1))) (f (u i))) +
      ∑ i ∈ Finset.Ico N n, edist (f (u (i + 1))) (f (u i)) =
    ∑ i ∈ Finset.Ico 0 (N - 1), edist (f (w (i + 1))) (f (w i)) + edist (f (u N)) (f (u (N - 1))) +
      ∑ i ∈ Finset.Ico N n, edist (f (w (1 + i + 1))) (f (w (1 + i)))
i₂ :
  ∑ i ∈ Finset.Ico 0 (N - 1), edist (f (w (i + 1))) (f (w i)) + edist (f (u N)) (f (u (N - 1))) +
      ∑ i ∈ Finset.Ico N n, edist (f (w (1 + i + 1))) (f (w (1 + i))) =
    ∑ i ∈ Finset.Ico 0 (N - 1), edist (f (w (i + 1))) (f (w i)) + edist (f (w (N + 1))) (f (w (N - 1))) +
      ∑ i ∈ Finset.Ico (N + 1) (n + 1), edist (f (w (i + 1))) (f (w i))
⊢ LE.le (α := ℝ≥0∞) (∑ i ∈ Finset.range n, edist (f (u (i + 1))) (f (u i)))
    (∑ j ∈ Finset.range (n + 1), edist (f (w (j + 1))) (f (w j)))
have i₃ : ∑ i ∈ Finset.Ico 0 (N - 1), edist (f (w (i + 1))) (f (w i)) + edist (f (w (N + 1))) (f (w (N - 1))) +
    ∑ i ∈ Finset.Ico (N + 1) (n + 1), edist (f (w (i + 1))) (f (w i)) ≤
  HAdd.hAdd (α := ℝ≥0∞) (∑ i ∈ Finset.Ico 0 (N - 1), edist (f (w (i + 1))) (f (w i)))
      (∑ i ∈ Finset.Ico (N - 1) (N + 1), edist (f (w (i + 1))) (f (w i))) +
    ∑ i ∈ Finset.Ico (N + 1) (n + 1), edist (f (w (i + 1))) (f (w i))
Tactic: calc
      (∑ i ∈ Finset.range n, edist (f (u (i + 1))) (f (u i))) =
          ((∑ i ∈ Finset.Ico 0 (N - 1), edist (f (u (i + 1))) (f (u i))) +
              ∑ i ∈ Finset.Ico (N - 1) N, edist (f (u (i + 1))) (f (u i))) +
            ∑ i ∈ Finset.Ico N n, edist (f (u (i + 1))) (f (u i)) := by
        rw [Finset.sum_Ico_consecutive, Finset.sum_Ico_consecutive, Finset.range_eq_Ico] <;> grind
      _ = (∑ i ∈ Finset.Ico 0 (N - 1), edist (f (w (i + 1))) (f (w i))) +
              edist (f (u N)) (f (u (N - 1))) +
            ∑ i ∈ Finset.Ico N n, edist (f (w (1 + i + 1))) (f (w (1 + i))) := by
        congr 1
        · congr 1
          · grind [Finset.sum_congr]
          · have A : N - 1 + 1 = N := Nat.succ_pred_eq_of_pos Npos
            have : Finset.Ico (N - 1) N = {N - 1} := by rw [← Nat.Ico_succ_singleton, A]
            simp only [this, A, Finset.sum_singleton]
        · grind [Finset.sum_congr]
      _ = (∑ i ∈ Finset.Ico 0 (N - 1), edist (f (w (i + 1))) (f (w i))) +
              edist (f (w (N + 1))) (f (w (N - 1))) +
            ∑ i ∈ Finset.Ico (N + 1) (n + 1), edist (f (w (i + 1))) (f (w i)) := by
        congr 1
        · grind
        · exact Finset.sum_Ico_add (fun i => edist (f (w (i + 1))) (f (w i))) N n 1
      _ ≤ ((∑ i ∈ Finset.Ico 0 (N - 1), edist (f (w (i + 1))) (f (w i))) +
              ∑ i ∈ Finset.Ico (N - 1) (N + 1), edist (f (w (i + 1))) (f (w i))) +
            ∑ i ∈ Finset.Ico (N + 1) (n + 1), edist (f (w (i + 1))) (f (w i)) := by
        refine add_le_add (add_le_add le_rfl ?_) le_rfl
        have A : N - 1 + 1 = N := Nat.succ_pred_eq_of_pos Npos
        have B : N - 1 + 1 < N + 1 := A.symm ▸ N.lt_succ_self
        have C : N - 1 < N + 1 := lt_of_le_of_lt N.pred_le N.lt_succ_self
        rw [Finset.sum_eq_sum_Ico_succ_bot C, Finset.sum_eq_sum_Ico_succ_bot B, A, Finset.Ico_self,
          Finset.sum_empty, add_zero, add_comm (edist _ _)]
        exact edist_triangle _ _ _
      _ = ∑ j ∈ Finset.range (n + 1), edist (f (w (j + 1))) (f (w j)) := by
        rw [Finset.sum_Ico_consecutive, Finset.sum_Ico_consecutive, Finset.range_eq_Ico] <;> grind