rw [Membership.mem, instMembershipSet]
α : Type u_1
G : group α
a b : α
a✝¹ : a ∈ { isMember := fun x => True }
a✝ : b ∈ { isMember := fun x => True }
⊢ a * b ∈ { isMember := fun x => True }
α : Type u_1
G : group α
a b : α
a✝¹ : a ∈ { isMember := fun x => True }
a✝ : b ∈ { isMember := fun x => True }
⊢ { mem := fun x s => Set.isMember x }.1 (a * b) { isMember := fun x => True }