feat: add forall_fin_zero and exists_fin_zero

Author: fgdorais

src/Init/Data/Fin/Lemmas.lean

@@ -915,16 +915,21 @@ theorem exists_fin_succ {P : Fin (n + 1) → Prop} : (∃ i, P i) ↔ P 0 ∨ 
   ⟨fun ⟨i, h⟩ => Fin.cases Or.inl (fun i hi => Or.inr ⟨i, hi⟩) i h, fun h =>
     (h.elim fun h => ⟨0, h⟩) fun ⟨i, hi⟩ => ⟨i.succ, hi⟩⟩
 
-theorem forall_fin_one {p : Fin 1 → Prop} : (∀ i, p i) ↔ p 0 :=
+@[simp] theorem forall_fin_zero {p : Fin 0 → Prop} : (∀ i, p i) ↔ True :=
+  ⟨fun _ => trivial, fun _ ⟨_, h⟩ => False.elim <| Nat.not_lt_zero _ h⟩
+
+theorem exists_fin_zero {p : Fin 0 → Prop} : (∃ i, p i) ↔ False := by simp
+
+@[simp] theorem forall_fin_one {p : Fin 1 → Prop} : (∀ i, p i) ↔ p 0 :=
   ⟨fun h => h _, fun h i => Subsingleton.elim i 0 ▸ h⟩
 
-theorem exists_fin_one {p : Fin 1 → Prop} : (∃ i, p i) ↔ p 0 :=
+@[simp] theorem exists_fin_one {p : Fin 1 → Prop} : (∃ i, p i) ↔ p 0 :=
   ⟨fun ⟨i, h⟩ => Subsingleton.elim i 0 ▸ h, fun h => ⟨_, h⟩⟩
 
-theorem forall_fin_two {p : Fin 2 → Prop} : (∀ i, p i) ↔ p 0 ∧ p 1 :=
+@[simp] theorem forall_fin_two {p : Fin 2 → Prop} : (∀ i, p i) ↔ p 0 ∧ p 1 :=
   forall_fin_succ.trans <| and_congr_right fun _ => forall_fin_one
 
-theorem exists_fin_two {p : Fin 2 → Prop} : (∃ i, p i) ↔ p 0 ∨ p 1 :=
+@[simp] theorem exists_fin_two {p : Fin 2 → Prop} : (∃ i, p i) ↔ p 0 ∨ p 1 :=
   exists_fin_succ.trans <| or_congr_right exists_fin_one
 
 theorem fin_two_eq_of_eq_zero_iff : ∀ {a b : Fin 2}, (a = 0 ↔ b = 0) → a = b := by