-- Lean 4 proof step from Mathlib.Topology.Sion (goal)
Goal:
E : Type u_1
F : Type u_2
β : Type u_3
inst : LinearOrder β
X : Set E
Y : Set F
f : E → F → β
inst_1 : TopologicalSpace E
inst_2 : AddCommGroup E
inst_3 : Module ℝ E
inst_4 : IsTopologicalAddGroup E
inst_5 : ContinuousSMul ℝ E
ne_X : X.Nonempty
kX : IsCompact X
hfy : ∀ y ∈ Y, LowerSemicontinuousOn.{u_1, u_3} (β := β) (fun (x : E) ↦ f x y) X
hfy' : ∀ y ∈ Y, QuasiconvexOn.{0, u_1, u_3} ℝ (β := β) X fun (x : E) ↦ f x y
inst_6 : TopologicalSpace F
inst_7 : AddCommGroup F
inst_8 : Module ℝ F
cY : Convex ℝ Y
hfx : ∀ x ∈ X, UpperSemicontinuousOn.{u_2, u_3} (β := β) (fun (y : F) ↦ f x y) Y
hfx' : ∀ x ∈ X, QuasiconcaveOn.{0, u_2, u_3} ℝ (β := β) Y fun (y : F) ↦ f x y
inst_9 : DenselyOrdered β
inst_10 : IsTopologicalAddGroup F
inst_11 : ContinuousSMul ℝ F
cX : Convex ℝ X
s : Set F
hs : s.Finite
hsY : s ⊆ Y
t : β
ht : ∀ x ∈ X, ∃ y ∈ s, t < f x y
⊢ ∃ y0 ∈ Y, ∀ x ∈ X, t < f x y0
have empty : ∀ {X : Set E} (ne_X : X.Nonempty) (kX : IsCompact X)
  (hfy : ∀ y ∈ Y, LowerSemicontinuousOn.{u_1, u_3} (β := β) (fun (x : E) ↦ f x y) X)
  (hfy' : ∀ y ∈ Y, QuasiconvexOn.{0, u_1, u_3} ℝ (β := β) X fun (x : E) ↦ f x y)
  (hfx : ∀ x ∈ X, UpperSemicontinuousOn.{u_2, u_3} (β := β) (fun (y : F) ↦ f x y) Y)
  (hfx' : ∀ x ∈ X, QuasiconcaveOn.{0, u_2, u_3} ℝ (β := β) Y fun (y : F) ↦ f x y) (cX : Convex ℝ X) (hsY : ∅ ⊆ Y)
  (ht : ∀ x ∈ X, ∃ (y : F), Membership.mem (γ := Set F) ∅ y ∧ t < f x y), ∃ y0 ∈ Y, ∀ x ∈ X, t < f x y0
Tactic: induction s, hs using Set.Finite.induction_on
    generalizing X with
  | empty => simpa using ht _ ne_X.choose_spec
  | @insert b s _ hs hrec =>  -- insert case
    have hb : (b ∈ Y) := hsY (mem_insert b s)
    let X' := X ∩ (fun x ↦ f x b) ⁻¹' Iic t
    rcases Set.eq_empty_or_nonempty X' with X'_e | X'_ne
    · -- When `X'` is empty, we may set `y0 := b`
      use b, hb
      simpa [X', Set.eq_empty_iff_forall_notMem] using X'_e
    -- the nonempty case
    have hX'X : X' ⊆ X := inter_subset_left
    have kX' : IsCompact X' := (hfy b hb).isCompact_inter_preimage_Iic kX t
    have cX' : Convex ℝ X' := hfy' b hb t
    specialize hrec X'_ne kX'
      (fun y hy ↦ LowerSemicontinuousOn.mono (hfy y hy) hX'X)
      (fun y hy ↦ cX'.quasiconvexOn_restrict (hfy' y hy) hX'X)
      (fun x hx' ↦ hfx x (hX'X hx'))
      (fun x hx' ↦ hfx' x (hX'X hx'))
      cX'
    have ht_lt : ∀ x ∈ X', ∃ y ∈ s, t < f x y := by
        /- otherwise, if  infi x ∈ X', supr y ∈ s f x y ≤ t
          then for every t' > t, there is x ∈ X' such that supr y ∈ s f x y ≤ t',
          since x ∈ X' and t ≤ t', we have supr y ∈ insert b s f x y  ≤ t',
          hence infi x ∈ X, supr y ∈ insert b s, f x y ≤ t',
          so that infi x ∈ X, supr y ∈ insert b s, f x y ≤ t. -/
      intro x hx
      obtain ⟨y, hy, h⟩ := ht x hx.1
      rcases hy with rfl | hy
      · simp [h.not_ge hx.2]
      · exact ⟨y, hy, h⟩
    obtain ⟨y1, hy1, hty1⟩ := hrec (by grw [subset_insert b s, hsY]) ht_lt
    refine exists_lt_iInf_of_lt_iInf_of_sup ne_X kX hfy hfy' cY hfx hfx' hb hy1 fun x hx ↦ ?_
    by_cases hx' : x ∈ X'
    · exact (hty1 x hx').trans_le (le_sup_right)
    · apply lt_of_lt_of_le _ (le_sup_left)
      rw [← not_le]
      exact fun h ↦ hx' ⟨hx, h⟩
Cleared hypotheses: X, ne_X, kX, hfy, hfy', hfx, hfx', cX, hs, hsY, ht