rw [preimage_image_quotientMk_stabilizer_eq_mul_mulStab hs s, ← coe_mul, mul_mulStab]
case inr
ι : Type u_1
α : Type u_2
inst✝¹ : CommGroup α
inst✝ : DecidableEq α
s✝ t : Finset α
a : α
s : Finset α
hs : s.Nonempty
⊢ QuotientGroup.mk ⁻¹' (QuotientGroup.mk '' ↑s) = ↑s
no goals