rw [Membership.mem, instMembershipSet] at ha
α : Type u_1
G : group α
H : Set α := trivial_subgroup G
hH : H = trivial_subgroup G
hmul_mem : ∀ (a b : α), a ∈ H → b ∈ H → a * b ∈ H
hone_mem : has_one.one ∈ H
a : α
ha : a ∈ H
⊢ group.inv a ∈ H
α : Type u_1
G : group α
H : Set α := trivial_subgroup G
hH : H = trivial_subgroup G
hmul_mem : ∀ (a b : α), a ∈ H → b ∈ H → a * b ∈ H
hone_mem : has_one.one ∈ H
a : α
ha : { mem := fun x s => Set.isMember x }.1 a H
⊢ group.inv a ∈ H