/-- An antitone function between (conditionally) complete linear ordered spaces sends a
`Filter.limsSup` to the `Filter.liminf` of the image if the function is continuous at the `limsSup`
(and the filter is bounded from above and below). -/
theorem Antitone.map_limsSup_of_continuousAt {F : Filter R} [NeBot F] {f : R → S}
    (f_decr : Antitone f) (f_cont : ContinuousAt f F.limsSup)
    (bdd_above : F.IsBounded (· ≤ ·) := by isBoundedDefault)
    (bdd_below : F.IsBounded (· ≥ ·) := by isBoundedDefault) :
    f F.limsSup = F.liminf f := by
  have cobdd : F.IsCobounded (· ≤ ·) := bdd_below.isCobounded_flip
  apply le_antisymm
  · rw [limsSup, f_decr.map_sInf_of_continuousAt' f_cont bdd_above cobdd]
    apply le_of_forall_lt
    intro c hc
    simp only [liminf, limsInf, eventually_map] at hc ⊢
    obtain ⟨d, hd, h'd⟩ :=
      exists_lt_of_lt_csSup (bdd_above.recOn fun x hx ↦ ⟨f x, Set.mem_image_of_mem f hx⟩) hc
    apply lt_csSup_of_lt ?_ ?_ h'd
    · exact (Antitone.isBoundedUnder_le_comp f_decr bdd_below).isCoboundedUnder_flip
    · rcases hd with ⟨e, ⟨he, fe_eq_d⟩⟩
      filter_upwards [he] with x hx using (fe_eq_d.symm ▸ f_decr hx)
  · by_cases h' : ∃ c, c < F.limsSup ∧ Set.Ioo c F.limsSup = ∅
    · rcases h' with ⟨c, c_lt, hc⟩
      have B : ∃ᶠ n in F, F.limsSup ≤ n := by
        apply (frequently_lt_of_lt_limsSup cobdd c_lt).mono
        intro x hx
        by_contra!
        have : (Set.Ioo c F.limsSup).Nonempty := ⟨x, ⟨hx, this⟩⟩
        simp only [hc, Set.not_nonempty_empty] at this
      apply liminf_le_of_frequently_le _ (bdd_above.isBoundedUnder f_decr)
      exact (B.mono fun x hx ↦ f_decr hx)
    push_neg at h'
    by_contra! H
    have not_bot : ¬ IsBot F.limsSup := fun maybe_bot ↦
      lt_irrefl (F.liminf f) <| lt_of_le_of_lt
        (liminf_le_of_frequently_le (frequently_of_forall (fun r ↦ f_decr (maybe_bot r)))
          (bdd_above.isBoundedUnder f_decr)) H
    obtain ⟨l, l_lt, h'l⟩ : ∃ l < F.limsSup, Set.Ioc l F.limsSup ⊆ { x : R | f x < F.liminf f }
    · apply exists_Ioc_subset_of_mem_nhds ((tendsto_order.1 f_cont.tendsto).2 _ H)
      simpa [IsBot] using not_bot
    obtain ⟨m, l_m, m_lt⟩ : (Set.Ioo l F.limsSup).Nonempty := by
      contrapose! h'
      exact ⟨l, l_lt, h'⟩
    have B : F.liminf f ≤ f m := by
      apply liminf_le_of_frequently_le _ _
      · 

Proof state:
ι : Type u_1
α : Type u_2
β : Type u_3
R : Type u_4
S : Type u_5
π : ι → Type u_6
F✝ : Filter ι
inst✝⁷ : NeBot F✝
inst✝⁶ : ConditionallyCompleteLinearOrder R
inst✝⁵ : TopologicalSpace R
inst✝⁴ : OrderTopology R
inst✝³ : ConditionallyCompleteLinearOrder S
inst✝² : TopologicalSpace S
inst✝¹ : OrderTopology S
F : Filter R
inst✝ : NeBot F
f : R → S
f_decr : Antitone f
f_cont : ContinuousAt f (limsSup F)
bdd_above : autoParam (IsBounded (fun x x_1 => x ≤ x_1) F) _auto✝
bdd_below : autoParam (IsBounded (fun x x_1 => x ≥ x_1) F) _auto✝
cobdd : IsCobounded (fun x x_1 => x ≤ x_1) F
h' : ∀ c < limsSup F, Set.Nonempty (Set.Ioo c (limsSup F))
H : f (limsSup F) < liminf f F
not_bot : ¬IsBot (limsSup F)
l : R
l_lt : l < limsSup F
h'l : Set.Ioc l (limsSup F) ⊆ {x | f x < liminf f F}
m : R
l_m : l < m
m_lt : m < limsSup F
⊢ ∃ᶠ (x : R) in F, f x ≤ f m

Next tactic:
apply (frequently_lt_of_lt_limsSup cobdd m_lt).mono