-- Mathlib/Combinatorics/SimpleGraph/Finsubgraph.lean
theorem nonempty_hom_of_forall_finite_subgraph_hom [Finite W]
    (h : ∀ G' : G.Subgraph, G'.verts.Finite → G'.coe →g F) : Nonempty (G →g F)

Proof state:
case intro.intro.refine_2
V : Type u
W : Type v
G : SimpleGraph V
F : SimpleGraph W
inst✝ : Finite W
h : (G' : G.Subgraph) → G'.verts.Finite → G'.coe →g F
val✝ : Fintype W
this✝ : ∀ (G' : G.Finsubgraphᵒᵖ), Nonempty ((G.finsubgraphHomFunctor F).obj G')
this : (G' : G.Finsubgraphᵒᵖ) → Fintype ((G.finsubgraphHomFunctor F).obj G')
u : (j : G.Finsubgraphᵒᵖ) → (G.finsubgraphHomFunctor F).obj j
hu : u ∈ (G.finsubgraphHomFunctor F).sections
v v' : V
e : G.Adj v v'
hv : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v)
hv' : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v')
⊢ (↑(Opposite.unop (Opposite.op (finsubgraphOfAdj e)))).coe.Adj ⟨v, ⋯⟩ ⟨v', ⋯⟩

Tactic:
simp [finsubgraphOfAdj]

Result:
no goals

Proof state:
case intro.intro.refine_2
V : Type u
W : Type v
G : SimpleGraph V
F : SimpleGraph W
inst✝ : Finite W
h : (G' : G.Subgraph) → G'.verts.Finite → G'.coe →g F
val✝ : Fintype W
this✝ : ∀ (G' : G.Finsubgraphᵒᵖ), Nonempty ((G.finsubgraphHomFunctor F).obj G')
this : (G' : G.Finsubgraphᵒᵖ) → Fintype ((G.finsubgraphHomFunctor F).obj G')
u : (j : G.Finsubgraphᵒᵖ) → (G.finsubgraphHomFunctor F).obj j
hu : u ∈ (G.finsubgraphHomFunctor F).sections
v v' : V
e : G.Adj v v'
hv : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v)
hv' : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v')
⊢ F.Adj ((u (Opposite.op (singletonFinsubgraph v))).toFun ⟨v, ⋯⟩)
    ((u (Opposite.op (singletonFinsubgraph v'))).toFun ⟨v', ⋯⟩)

Tactic:
rw [← hu hv, ← hu hv']

Result:
case intro.intro.refine_2
V : Type u
W : Type v
G : SimpleGraph V
F : SimpleGraph W
inst✝ : Finite W
h : (G' : G.Subgraph) → G'.verts.Finite → G'.coe →g F
val✝ : Fintype W
this✝ : ∀ (G' : G.Finsubgraphᵒᵖ), Nonempty ((G.finsubgraphHomFunctor F).obj G')
this : (G' : G.Finsubgraphᵒᵖ) → Fintype ((G.finsubgraphHomFunctor F).obj G')
u : (j : G.Finsubgraphᵒᵖ) → (G.finsubgraphHomFunctor F).obj j
hu : u ∈ (G.finsubgraphHomFunctor F).sections
v v' : V
e : G.Adj v v'
hv : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v)
hv' : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v')
⊢ F.Adj (((G.finsubgraphHomFunctor F).map hv (u (Opposite.op (finsubgraphOfAdj e)))).toFun ⟨v, ⋯⟩)
    (((G.finsubgraphHomFunctor F).map hv' (u (Opposite.op (finsubgraphOfAdj e)))).toFun ⟨v', ⋯⟩)

Proof state:
case intro.intro.refine_2
V : Type u
W : Type v
G : SimpleGraph V
F : SimpleGraph W
inst✝ : Finite W
h : (G' : G.Subgraph) → G'.verts.Finite → G'.coe →g F
val✝ : Fintype W
this✝ : ∀ (G' : G.Finsubgraphᵒᵖ), Nonempty ((G.finsubgraphHomFunctor F).obj G')
this : (G' : G.Finsubgraphᵒᵖ) → Fintype ((G.finsubgraphHomFunctor F).obj G')
u : (j : G.Finsubgraphᵒᵖ) → (G.finsubgraphHomFunctor F).obj j
hu : u ∈ (G.finsubgraphHomFunctor F).sections
v v' : V
e : G.Adj v v'
hv : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v)
⊢ F.Adj ((u (Opposite.op (singletonFinsubgraph v))).toFun ⟨v, ⋯⟩)
    ((u (Opposite.op (singletonFinsubgraph v'))).toFun ⟨v', ⋯⟩)

Tactic:
have hv' : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v') :=
  Quiver.Hom.op (CategoryTheory.homOfLE singletonFinsubgraph_le_adj_right)

Result:
case intro.intro.refine_2
V : Type u
W : Type v
G : SimpleGraph V
F : SimpleGraph W
inst✝ : Finite W
h : (G' : G.Subgraph) → G'.verts.Finite → G'.coe →g F
val✝ : Fintype W
this✝ : ∀ (G' : G.Finsubgraphᵒᵖ), Nonempty ((G.finsubgraphHomFunctor F).obj G')
this : (G' : G.Finsubgraphᵒᵖ) → Fintype ((G.finsubgraphHomFunctor F).obj G')
u : (j : G.Finsubgraphᵒᵖ) → (G.finsubgraphHomFunctor F).obj j
hu : u ∈ (G.finsubgraphHomFunctor F).sections
v v' : V
e : G.Adj v v'
hv : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v)
hv' : Opposite.op (finsubgraphOfAdj e) ⟶ Opposite.op (singletonFinsubgraph v')
⊢ F.Adj ((u (Opposite.op (singletonFinsubgraph v))).toFun ⟨v, ⋯⟩)
    ((u (Opposite.op (singletonFinsubgraph v'))).toFun ⟨v', ⋯⟩)