-- 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
n : ℕ
u : ℕ → α
u_mono : Monotone u
u_mem : ∀ (i : ℕ), u i ∈ s
this :
  ∃ (p : (n : ℕ) × { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s }),
    (↑(p.snd (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) : ℕ → α)
          (p.fst (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) =
        u n ∧
      Eq (α := ℝ≥0∞) (∑ x ∈ Finset.range n, edist (f (u (x + 1))) (f (u x)))
        (∑ i ∈ Finset.range (p.fst (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })),
          edist
            (f
              ((↑(p.snd (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) : ℕ → α)
                (i + 1)))
            (f
              ((↑(p.snd (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) : ℕ → α)
                i)))
⊢ LE.le (α := ℝ≥0∞)
    (∑ i ∈ Finset.range (n, Subtype.mk (p := fun (u : ℕ → α) ↦ Monotone u ∧ ∀ (i : ℕ), u i ∈ s) u ⋯).1,
      edist (f ((↑(n, Subtype.mk (p := fun (u : ℕ → α) ↦ Monotone u ∧ ∀ (i : ℕ), u i ∈ s) u ⋯).2 : ℕ → α) (i + 1)))
        (f ((↑(n, Subtype.mk (p := fun (u : ℕ → α) ↦ Monotone u ∧ ∀ (i : ℕ), u i ∈ s) u ⋯).2 : ℕ → α) i)))
    (⨆ (p : (n : ℕ) × { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s }),
      ∑ i ∈ Finset.range (p.fst (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })),
        edist
          (f
            ((↑(p.snd (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) : ℕ → α)
              (i + 1)))
          (f
            ((↑(p.snd (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) : ℕ → α) i)))
have a : ∀ (p : (n : ℕ) × { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })
  (hp :
    Eq (α := ℝ≥0∞) (∑ x ∈ Finset.range n, edist (f (u (x + 1))) (f (u x)))
      (∑ i ∈ Finset.range (p.fst (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })),
        edist
          (f
            ((↑(p.snd (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) : ℕ → α)
              (i + 1)))
          (f
            ((↑(p.snd (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) : ℕ → α)
              i)))),
  LE.le (α := ℝ≥0∞)
    (∑ i ∈ Finset.range (n, Subtype.mk (p := fun (u : ℕ → α) ↦ Monotone u ∧ ∀ (i : ℕ), u i ∈ s) u sorry).1,
      edist (f ((↑(n, Subtype.mk (p := fun (u : ℕ → α) ↦ Monotone u ∧ ∀ (i : ℕ), u i ∈ s) u sorry).2 : ℕ → α) (i + 1)))
        (f ((↑(n, Subtype.mk (p := fun (u : ℕ → α) ↦ Monotone u ∧ ∀ (i : ℕ), u i ∈ s) u sorry).2 : ℕ → α) i)))
    (⨆ (p : (n : ℕ) × { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s }),
      ∑ i ∈ Finset.range (p.fst (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })),
        edist
          (f
            ((↑(p.snd (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) : ℕ → α)
              (i + 1)))
          (f
            ((↑(p.snd (β := fun (n : ℕ) ↦ { u : ℕ → α // StrictMonoOn u (Iic n) ∧ ∀ i ∈ Iic n, u i ∈ s })) : ℕ → α) i)))
Tactic: rcases this with ⟨p, -, hp⟩
Cleared hypotheses: this