feat(group_theory/subgroup): add closure_eq_bot_iff (#5055)

Add missing lemma

src/group_theory/subgroup.lean

@@ -560,6 +560,11 @@ lemma closure_union (s t : set G) : closure (s ∪ t) = closure s ⊔ closure t
 lemma closure_Union {ι} (s : ι → set G) : closure (⋃ i, s i) = ⨆ i, closure (s i) :=
 (subgroup.gi G).gc.l_supr
 
+@[to_additive]
+lemma closure_eq_bot_iff (G : Type*) [group G] (S : set G) :
+  closure S = ⊥ ↔ S ⊆ {1} :=
+by { rw [← le_bot_iff], exact closure_le _}
+
 /-- The subgroup generated by an element of a group equals the set of integer number powers of
     the element. -/
 lemma mem_closure_singleton {x y : G} : y ∈ closure ({x} : set G) ↔ ∃ n : ℤ, x ^ n = y :=