-- Mathlib/Combinatorics/Colex.lean
/-- Initial segments are nested in some way. In particular, if they're the same size they're equal.
-/
lemma IsInitSeg.total (h₁ : IsInitSeg 𝒜₁ r) (h₂ : IsInitSeg 𝒜₂ r) : 𝒜₁ ⊆ 𝒜₂ ∨ 𝒜₂ ⊆ 𝒜₁

Proof state:
case inl
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
s : Finset α
hs : s ∈ 𝒜₁ ∧ s ∉ 𝒜₂
t : Finset α
ht : t ∈ 𝒜₂ ∧ t ∉ 𝒜₁
hst : { ofColex := s } < { ofColex := t }
⊢ False

Tactic:
exact hs.2 <| h₂.2 ht.1 ⟨hst, h₁.1 hs.1⟩

Result:
no goals

Proof state:
case inr.inr
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
s : Finset α
hs : s ∈ 𝒜₁ ∧ s ∉ 𝒜₂
t : Finset α
ht : t ∈ 𝒜₂ ∧ t ∉ 𝒜₁
hts : { ofColex := t } < { ofColex := s }
⊢ False

Tactic:
exact ht.2 <| h₁.2 hs.1 ⟨hts, h₂.1 ht.1⟩

Result:
no goals

Proof state:
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
⊢ ¬(𝒜₁ \ 𝒜₂).Nonempty ∨ ¬(𝒜₂ \ 𝒜₁).Nonempty

Tactic:
by_contra! h

Result:
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
⊢ False

Proof state:
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
⊢ 𝒜₁ ⊆ 𝒜₂ ∨ 𝒜₂ ⊆ 𝒜₁

Tactic:
simp_rw [← sdiff_eq_empty_iff_subset, ← not_nonempty_iff_eq_empty]

Result:
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
⊢ ¬(𝒜₁ \ 𝒜₂).Nonempty ∨ ¬(𝒜₂ \ 𝒜₁).Nonempty

Proof state:
case inr.inl
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
s : Finset α
hs : s ∈ 𝒜₁ ∧ s ∉ 𝒜₂
t : Finset α
ht : t ∈ 𝒜₂ ∧ t ∉ 𝒜₁
hst : s = t
⊢ False

Tactic:
exact ht.2 <| hst ▸ hs.1

Result:
no goals

Proof state:
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
⊢ False

Tactic:
have ⟨⟨s, hs⟩, t, ht⟩ := h

Result:
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
s : Finset α
hs : s ∈ 𝒜₁ \ 𝒜₂
t : Finset α
ht : t ∈ 𝒜₂ \ 𝒜₁
⊢ False

Proof state:
case inr.inl
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
s : Finset α
hs : s ∈ 𝒜₁ ∧ s ∉ 𝒜₂
t : Finset α
ht : t ∈ 𝒜₂ ∧ t ∉ 𝒜₁
hst : { ofColex := s } = { ofColex := t }
⊢ False

Tactic:
simp only [toColex.injEq] at hst

Result:
case inr.inl
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
s : Finset α
hs : s ∈ 𝒜₁ ∧ s ∉ 𝒜₂
t : Finset α
ht : t ∈ 𝒜₂ ∧ t ∉ 𝒜₁
hst : s = t
⊢ False

Proof state:
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
s : Finset α
hs : s ∈ 𝒜₁ \ 𝒜₂
t : Finset α
ht : t ∈ 𝒜₂ \ 𝒜₁
⊢ False

Tactic:
rw [mem_sdiff] at hs ht

Result:
α : Type u_1
inst✝ : LinearOrder α
𝒜₁ 𝒜₂ : Finset (Finset α)
r : ℕ
h₁ : IsInitSeg 𝒜₁ r
h₂ : IsInitSeg 𝒜₂ r
h : (𝒜₁ \ 𝒜₂).Nonempty ∧ (𝒜₂ \ 𝒜₁).Nonempty
s : Finset α
hs : s ∈ 𝒜₁ ∧ s ∉ 𝒜₂
t : Finset α
ht : t ∈ 𝒜₂ ∧ t ∉ 𝒜₁
⊢ False