have := QuotientGroup.preimageMkEquivSubgroupProdSet (stabilizer α (s * t))
  ((s +ˢ stabilizer α (s * t)) * (t +ˢ stabilizer α (s * t)))
case inr.inr
ι : Type u_1
α : Type u_2
inst✝¹ : CommGroup α
inst✝ : DecidableEq α
s✝ t✝ : Finset α
a : α
s t : Finset α
hs : s.Nonempty
ht : t.Nonempty
⊢ (s * t).mulStab.card * (image QuotientGroup.mk s * image QuotientGroup.mk t).card = (s * t).card
case inr.inr
ι : Type u_1
α : Type u_2
inst✝¹ : CommGroup α
inst✝ : DecidableEq α
s✝ t✝ : Finset α
a : α
s t : Finset α
hs : s.Nonempty
ht : t.Nonempty
this :
  ↑(QuotientGroup.mk ⁻¹' (QuotientGroup.mk '' ↑s * QuotientGroup.mk '' ↑t)) ≃
    ↥(stabilizer α (s * t)) × ↑(QuotientGroup.mk '' ↑s * QuotientGroup.mk '' ↑t)
⊢ (s * t).mulStab.card * (image QuotientGroup.mk s * image QuotientGroup.mk t).card = (s * t).card