simp at ha
α : Type u_1
G : group α
H : Set α := trivial_subgroup G
hH : H = trivial_subgroup G
a b : α
ha : { mem := fun x s => Set.isMember x }.1 a H
hb : b ∈ H
⊢ a * b ∈ H
α : Type u_1
G : group α
H : Set α := trivial_subgroup G
hH : H = trivial_subgroup G
a b : α
ha : Set.isMember a
hb : b ∈ H
⊢ a * b ∈ H