exact {
  mul_mem := hmul_mem
  one_mem := hone_mem
  inv_mem := hinv_mem
}
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
hinv_mem : ∀ (a : α), a ∈ H → group.inv a ∈ H
⊢ subgroup G H
no goals