/-- Given two pairs of distinct points on the same line, such that the vectors between those
pairs of points are on the same ray (oriented in the same direction on that line), and a fifth
point, the angles at the fifth point between each of those two pairs of points have the same
sign. -/
theorem _root_.Collinear.oangle_sign_of_sameRay_vsub {p₁ p₂ p₃ p₄ : P} (p₅ : P) (hp₁p₂ : p₁ ≠ p₂)
    (hp₃p₄ : p₃ ≠ p₄) (hc : Collinear ℝ ({p₁, p₂, p₃, p₄} : Set P))
    (hr : SameRay ℝ (p₂ -ᵥ p₁) (p₄ -ᵥ p₃)) : (∡ p₁ p₅ p₂).sign = (∡ p₃ p₅ p₄).sign := by
  by_cases hc₅₁₂ : Collinear ℝ ({p₅, p₁, p₂} : Set P)
  · have hc₅₁₂₃₄ : Collinear ℝ ({p₅, p₁, p₂, p₃, p₄} : Set P) :=
      (hc.collinear_insert_iff_of_ne (Set.mem_insert _ _)
        (Set.mem_insert_of_mem _ (Set.mem_insert _ _)) hp₁p₂).2 hc₅₁₂
    have hc₅₃₄ : Collinear ℝ ({p₅, p₃, p₄} : Set P) :=
      (hc.collinear_insert_iff_of_ne
        (Set.mem_insert_of_mem _ (Set.mem_insert_of_mem _ (Set.mem_insert _ _)))
        (Set.mem_insert_of_mem _ (Set.mem_insert_of_mem _ (Set.mem_insert_of_mem _
          (Set.mem_singleton _)))) hp₃p₄).1 hc₅₁₂₃₄
    rw [Set.insert_comm] at hc₅₁₂ hc₅₃₄
    have hs₁₅₂ := oangle_eq_zero_or_eq_pi_iff_collinear.2 hc₅₁₂
    have hs₃₅₄ := oangle_eq_zero_or_eq_pi_iff_collinear.2 hc₅₃₄
    rw [← Real.Angle.sign_eq_zero_iff] at hs₁₅₂ hs₃₅₄
    rw [hs₁₅₂, hs₃₅₄]
  · let s : Set (P × P × P) :=
      (fun x : line[ℝ, p₁, p₂] × V => (x.1, p₅, x.2 +ᵥ (x.1 : P))) ''
        Set.univ ×ˢ {v | SameRay ℝ (p₂ -ᵥ p₁) v ∧ v ≠ 0}
    have hco : IsConnected s :=
      haveI : ConnectedSpace line[ℝ, p₁, p₂] := AddTorsor.connectedSpace _ _
      (isConnected_univ.prod (isConnected_setOf_sameRay_and_ne_zero
        (vsub_ne_zero.2 hp₁p₂.symm))).image _
        (continuous_fst.subtype_val.prod_mk (continuous_const.prod_mk
          (continuous_snd.vadd continuous_fst.subtype_val))).continuousOn
    have hf : ContinuousOn (fun p : P × P × P => ∡ p.1 p.2.1 p.2.2) s := by
      refine' ContinuousAt.continuousOn fun p hp => continuousAt_oangle _ _
      all_goals
        simp_rw [Set.mem_image, Set.mem_prod, Set.mem_univ, true_and_iff, Prod.ext_iff] at hp
        obtain ⟨q₁, q₅, q₂⟩ := p
        dsimp only at hp ⊢
        obtain ⟨⟨⟨q, hq⟩, v⟩, hv, rfl, rfl, rfl⟩ := hp
        dsimp only [Subtype.coe_mk, Set.mem_setOf] at hv ⊢
        obtain ⟨hvr, -⟩ := hv
        rintro rfl
        refine' hc₅₁₂ ((collinear_insert_iff_of_mem_affineSpan _).2 (collinear_pair _ _ _))
      · exact hq
      · refine' vadd_mem_of_mem_direction _ hq
        rw [← exists_nonneg_left_iff_sameRay (vsub_ne_zero.2 hp₁p₂.symm)] at hvr
        obtain ⟨r, -, rfl⟩ := hvr
        rw [direction_affineSpan]
        exact smul_vsub_rev_mem_vectorSpan_pair _ _ _
    have hsp : ∀ p : P × P × P, p ∈ s → ∡ p.1 p.2.1 p.2.2 ≠ 0 ∧ ∡ p.1 p.2.1 p.2.2 ≠ π := by
      intro p hp
      simp_rw [Set.mem_image, Set.mem_prod, Set.mem_setOf, Set.mem_univ, true_and_iff,
        Prod.ext_iff] at hp
      obtain ⟨q₁, q₅, q₂⟩ := p
      dsimp only at hp ⊢
      obtain ⟨⟨⟨q, hq⟩, v⟩, hv, rfl, rfl, rfl⟩ := hp
      dsimp only [Subtype.coe_mk, Set.mem_setOf] at hv ⊢
      obtain ⟨hvr, hv0⟩ := hv
      rw [← exists_nonneg_left_iff_sameRay (vsub_ne_zero.2 hp₁p₂.symm)] at hvr
      obtain ⟨r, -, rfl⟩ := hvr
      change q ∈ line[ℝ, p₁, p₂] at hq
      rw [oangle_ne_zero_and_ne_pi_iff_affineIndependent]
      refine' affineIndependent_of_ne_of_mem_of_not_mem_of_mem _ hq
          (fun h => hc₅₁₂ ((collinear_insert_iff_of_mem_affineSpan h).2 (collinear_pair _ _ _))) _
      · rwa [← @vsub_ne_zero V, vsub_vadd_eq_vsub_sub, vsub_self, zero_sub, neg_ne_zero]
      · refine' vadd_mem_of_mem_direction _ hq
        rw [direction_affineSpan]
        exact smul_vsub_rev_mem_vectorSpan_pair _ _ _
    

Proof state:
case neg
V : Type u_1
P : Type u_2
inst✝⁴ : NormedAddCommGroup V
inst✝³ : InnerProductSpace ℝ V
inst✝² : MetricSpace P
inst✝¹ : NormedAddTorsor V P
hd2 : Fact (finrank ℝ V = 2)
inst✝ : Module.Oriented ℝ V (Fin 2)
p₁ p₂ p₃ p₄ p₅ : P
hp₁p₂ : p₁ ≠ p₂
hp₃p₄ : p₃ ≠ p₄
hc : Collinear ℝ {p₁, p₂, p₃, p₄}
hr : SameRay ℝ (p₂ -ᵥ p₁) (p₄ -ᵥ p₃)
hc₅₁₂ : ¬Collinear ℝ {p₅, p₁, p₂}
s : Set (P × P × P) := (fun x => (↑x.1, p₅, x.2 +ᵥ ↑x.1)) '' Set.univ ×ˢ {v | SameRay ℝ (p₂ -ᵥ p₁) v ∧ v ≠ 0}
hco : IsConnected s
hf : ContinuousOn (fun p => ∡ p.1 p.2.1 p.2.2) s
hsp : ∀ p ∈ s, ∡ p.1 p.2.1 p.2.2 ≠ 0 ∧ ∡ p.1 p.2.1 p.2.2 ≠ ↑π
⊢ Real.Angle.sign (∡ p₁ p₅ p₂) = Real.Angle.sign (∡ p₃ p₅ p₄)

Next tactic:
have hp₁p₂s : (p₁, p₅, p₂) ∈ s := by
      simp_rw [Set.mem_image, Set.mem_prod, Set.mem_setOf, Set.mem_univ, true_and_iff, Prod.ext_iff]
      refine' ⟨⟨⟨p₁, left_mem_affineSpan_pair ℝ _ _⟩, p₂ -ᵥ p₁⟩,
        ⟨SameRay.rfl, vsub_ne_zero.2 hp₁p₂.symm⟩, _⟩
      simp