Module: Mathlib.Combinatorics.HalesJewett

Goal state before:
pp: ⊢ ∀ {α : Type u} [Fintype α],
    (∀ (κ : Type (max v u)) [Finite κ], ∃ ι x, ∀ (C : (ι → α) → κ), ∃ l, IsMono C l) →
      ∀ (κ : Type (max v u)) [Finite κ], ∃ ι x, ∀ (C : (ι → Option α) → κ), ∃ l, IsMono C l

Tactic:
intro α _ ihα κ _
    cases nonempty_fintype κ
    -- Later we'll need `α` to be nonempty. So we first deal with the trivial case where `α` is
    -- empty.
    -- Then `Option α` has only one element, so any line is monochromatic.
    by_cases h : Nonempty α
    case neg =>
      refine ⟨Unit, inferInstance, fun C => ⟨diagonal _ Unit, C fun _ => none, ?_⟩⟩
      rintro (_ | ⟨a⟩)
      · rfl
      · exact (h ⟨a⟩).elim
    -- The key idea is to show that for every `r`, in high dimension we can either find
    -- `r` color focused lines or a monochromatic line.
    suffices key :
      ∀ r : ℕ,
        ∃ (ι : Type) (_ : Fintype ι),
          ∀ C : (ι → Option α) → κ,
            (∃ s : ColorFocused C, Multiset.card s.lines = r) ∨ ∃ l, IsMono C l by
      -- Given the key claim, we simply take `r = |κ| + 1`. We cannot have this many distinct colors
      -- so we must be in the second case, where there is a monochromatic line.
      obtain ⟨ι, _inst, hι⟩ := key (Fintype.card κ + 1)
      refine ⟨ι, _inst, fun C => (hι C).resolve_left ?_⟩
      rintro ⟨s, sr⟩
      apply Nat.not_succ_le_self (Fintype.card κ)
      rw [← Nat.add_one, ← sr, ← Multiset.card_map, ← Finset.card_mk]
      exact Finset.card_le_univ ⟨_, s.distinct_colors⟩
    -- We now prove the key claim, by induction on `r`.
    intro r
    induction r with
    -- The base case `r = 0` is trivial as the empty collection is color-focused.
    | zero => exact ⟨Empty, inferInstance, fun C => Or.inl ⟨default, Multiset.card_zero⟩⟩
    | succ r ihr =>
    -- Supposing the key claim holds for `r`, we need to show it for `r+1`. First pick a high
    -- enough dimension `ι` for `r`.
    obtain ⟨ι, _inst, hι⟩ := ihr
    -- Then since the theorem holds for `α` with any number of colors, pick a dimension `ι'` such
    -- that `ι' → α` always has a monochromatic line whenever it is `(ι → Option α) → κ`-colored.
    specialize ihα ((ι → Option α) → κ)
    obtain ⟨ι', _inst, hι'⟩ := ihα
    -- We claim that `ι ⊕ ι'` works for `Option α` and `κ`-coloring.
    refine ⟨ι ⊕ ι', inferInstance, ?_⟩
    intro C
    -- A `κ`-coloring of `ι ⊕ ι' → Option α` induces an `(ι → Option α) → κ`-coloring of `ι' → α`.
    specialize hι' fun v' v => C (Sum.elim v (some ∘ v'))
    -- By choice of `ι'` this coloring has a monochromatic line `l'` with color class `C'`, where
    -- `C'` is a `κ`-coloring of `ι → α`.
    obtain ⟨l', C', hl'⟩ := hι'
    -- If `C'` has a monochromatic line, then so does `C`. We use this in two places below.
    have mono_of_mono : (∃ l, IsMono C' l) → ∃ l, IsMono C l := by
      rintro ⟨l, c, hl⟩
      refine ⟨l.horizontal (some ∘ l' (Classical.arbitrary α)), c, fun x => ?_⟩
      rw [Line.horizontal_apply, ← hl, ← hl']
    -- By choice of `ι`, `C'` either has `r` color-focused lines or a monochromatic line.
    specialize hι C'
    rcases hι with (⟨s, sr⟩ | h)
    on_goal 2 => exact Or.inr (mono_of_mono h)
    -- Here we assume `C'` has `r` color focused lines. We split into cases depending on whether
    -- one of these `r` lines has the same color as the focus point.
    by_cases h : ∃ p ∈ s.lines, (p : AlmostMono _).color = C' s.focus
    -- If so then this is a `C'`-monochromatic line and we are done.
    · obtain ⟨p, p_mem, hp⟩ := h
      refine Or.inr (mono_of_mono ⟨p.line, p.color, ?_⟩)
      rintro (_ | _)
      · rw [hp, s.is_focused p p_mem]
      · apply p.has_color
    -- If not, we get `r+1` color focused lines by taking the product of the `r` lines with `l'`
    -- and adding to this the vertical line obtained by the focus point and `l`.
    refine Or.inl ⟨⟨(s.lines.map ?_).cons ⟨(l'.map some).vertical s.focus, C' s.focus, fun x => ?_⟩,
            Sum.elim s.focus (l'.map some none), ?_, ?_⟩, ?_⟩
    -- The product lines are almost monochromatic.
    · refine fun p => ⟨p.line.prod (l'.map some), p.color, fun x => ?_⟩
      rw [Line.prod_apply, Line.map_apply, ← p.has_color, ← congr_fun (hl' x)]
    -- The vertical line is almost monochromatic.
    · rw [vertical_apply, ← congr_fun (hl' x), Line.map_apply]
    -- Our `r+1` lines have the same endpoint.
    · simp_rw [Multiset.mem_cons, Multiset.mem_map]
      rintro _ (rfl | ⟨q, hq, rfl⟩)
      · simp only [vertical_apply]
      · simp only [prod_apply, s.is_focused q hq]
    -- Our `r+1` lines have distinct colors (this is why we needed to split into cases above).
    · rw [Multiset.map_cons, Multiset.map_map, Multiset.nodup_cons, Multiset.mem_map]
      exact ⟨h, s.distinct_colors⟩
    -- Finally, we really do have `r+1` lines!
    · rw [Multiset.card_cons, Multiset.card_map, sr]

Goal state after: