Module: Mathlib.Combinatorics.SimpleGraph.Tutte

Goal state before:
pp: case neg
V : Type u_1
G : SimpleGraph V
inst✝ : Finite V
x a b c : V
M1 : (G ⊔ edge x b).Subgraph
M2 : (G ⊔ edge a c).Subgraph
hxa : G.Adj x a
hab : G.Adj a b
hnGxb : ¬G.Adj x b
hnGac : ¬G.Adj a c
hnxb : x ≠ b
hnxc : x ≠ c
hnac : a ≠ c
hnbc : b ≠ c
hM1 : M1.IsPerfectMatching
hM2 : M2.IsPerfectMatching
hM1xb : M1.Adj x b
hM2ac : M2.Adj a c
hM1nac : ¬M1.Adj a c
hsupG : G ⊔ edge x b ⊔ (G ⊔ edge a c) = G ⊔ edge a c ⊔ edge x b
cycles : SimpleGraph V := M1.spanningCoe ∆ M2.spanningCoe
hcalt : cycles.IsAlternating M2.spanningCoe
hcycles : (M1.spanningCoe ∆ M2.spanningCoe).IsCycles
hcac : cycles.Adj a c
hM1sub : M1.spanningCoe ≤ G ⊔ edge x b
hM2sub : M2.spanningCoe ≤ G ⊔ edge a c
induce_le : (cycles.connectedComponentMk c).toSimpleGraph.spanningCoe ≤ G ⊔ edge a c ⊔ edge x b
hxc : x ∈ (cycles.connectedComponentMk c).supp
hacc : a ∈ (cycles.connectedComponentMk c).supp
this : (G : SimpleGraph V) → G.LocallyFinite
hnM2 : ∀ (x' : V), x' ≠ c → ¬M2.Adj x' a
⊢ ∃ G', G'.IsAlternating M2.spanningCoe ∧ G'.IsCycles ∧ ¬G'.Adj x b ∧ G'.Adj a c ∧ G' ≤ G ⊔ edge a c

Tactic:
obtain ⟨x', hx', p, hp, hpac, hnpxb⟩ :
      ∃ x' ∈ ({x, b} : Finset V), ∃ (p : cycles.Walk a x'), p.IsPath ∧
        p.toSubgraph.Adj a c ∧ ¬p.toSubgraph.Adj x b := by
    obtain ⟨p, hp⟩ := hcycles.exists_cycle_toSubgraph_verts_eq_connectedComponentSupp hacc
      ⟨_, hcac⟩
    obtain ⟨p', hp'⟩ := hp.1.exists_isCycle_snd_verts_eq (by
      rwa [hp.1.adj_toSubgraph_iff_of_isCycles hcycles (hp.2 ▸ hacc)])
    obtain ⟨x', hx', hx'p, htw⟩ := Walk.exists_mem_support_forall_mem_support_imp_eq {x, b} <| by
      use x
      simp only [Finset.mem_filter, Finset.mem_insert, Finset.mem_singleton, true_or, true_and]
      rwa [← @Walk.mem_verts_toSubgraph, hp'.2.2, hp.2]
    refine ⟨x', hx', p'.takeUntil x' hx'p, hp'.1.isPath_takeUntil hx'p, ?_, fun h ↦ ?_⟩; swap
    · simp [htw _ (by simp) (Walk.mem_support_of_adj_toSubgraph h.symm),
        htw _ (by simp) (Walk.mem_support_of_adj_toSubgraph h)] at hnxb
    have : (p'.takeUntil x' hx'p).toSubgraph.Adj a (p'.takeUntil x' hx'p).snd := by
      apply Walk.toSubgraph_adj_snd
      rw [Walk.nil_takeUntil]
      grind
    rwa [Walk.snd_takeUntil, hp'.2.1] at this
    simp only [Finset.mem_insert, Finset.mem_singleton] at hx'
    obtain rfl | rfl := hx'
    exacts [hxa.ne, hab.ne.symm]

Goal state after:
case neg
V : Type u_1
G : SimpleGraph V
inst✝ : Finite V
x a b c : V
M1 : (G ⊔ edge x b).Subgraph
M2 : (G ⊔ edge a c).Subgraph
hxa : G.Adj x a
hab : G.Adj a b
hnGxb : ¬G.Adj x b
hnGac : ¬G.Adj a c
hnxb : x ≠ b
hnxc : x ≠ c
hnac : a ≠ c
hnbc : b ≠ c
hM1 : M1.IsPerfectMatching
hM2 : M2.IsPerfectMatching
hM1xb : M1.Adj x b
hM2ac : M2.Adj a c
hM1nac : ¬M1.Adj a c
hsupG : G ⊔ edge x b ⊔ (G ⊔ edge a c) = G ⊔ edge a c ⊔ edge x b
cycles : SimpleGraph V := M1.spanningCoe ∆ M2.spanningCoe
hcalt : cycles.IsAlternating M2.spanningCoe
hcycles : (M1.spanningCoe ∆ M2.spanningCoe).IsCycles
hcac : cycles.Adj a c
hM1sub : M1.spanningCoe ≤ G ⊔ edge x b
hM2sub : M2.spanningCoe ≤ G ⊔ edge a c
induce_le : (cycles.connectedComponentMk c).toSimpleGraph.spanningCoe ≤ G ⊔ edge a c ⊔ edge x b
hxc : x ∈ (cycles.connectedComponentMk c).supp
hacc : a ∈ (cycles.connectedComponentMk c).supp
this : (G : SimpleGraph V) → G.LocallyFinite
hnM2 : ∀ (x' : V), x' ≠ c → ¬M2.Adj x' a
x' : V
hx' : x' ∈ {x, b}
p : cycles.Walk a x'
hp : p.IsPath
hpac : p.toSubgraph.Adj a c
hnpxb : ¬p.toSubgraph.Adj x b
⊢ ∃ G', G'.IsAlternating M2.spanningCoe ∧ G'.IsCycles ∧ ¬G'.Adj x b ∧ G'.Adj a c ∧ G' ≤ G ⊔ edge a c