rw [← Int.ofNat_sub (card_le_card (mul_subset_mul_left hst)), ←
  card_sdiff (mul_subset_mul_left hst)]
α : Type u_1
inst✝¹ : CommGroup α
inst✝ : DecidableEq α
s✝ s' t✝ t' C✝ : Finset α
a✝ b a : α
s t C : Finset α
has : a ∈ s
hst : (s ∩ a • C.mulStab * (t ∩ a • C.mulStab)).mulStab ⊆ C.mulStab
hC : C.Nonempty
⊢ ↑(a • C.mulStab \
          ((s ∩ a • C.mulStab ∪ t ∩ a • C.mulStab) * (s ∩ a • C.mulStab * (t ∩ a • C.mulStab)).mulStab)).card ≤
    ↑((s ∪ t) * C.mulStab).card - ↑((s ∪ t) * (s ∩ a • C.mulStab * (t ∩ a • C.mulStab)).mulStab).card
α : Type u_1
inst✝¹ : CommGroup α
inst✝ : DecidableEq α
s✝ s' t✝ t' C✝ : Finset α
a✝ b a : α
s t C : Finset α
has : a ∈ s
hst : (s ∩ a • C.mulStab * (t ∩ a • C.mulStab)).mulStab ⊆ C.mulStab
hC : C.Nonempty
⊢ ↑(a • C.mulStab \
          ((s ∩ a • C.mulStab ∪ t ∩ a • C.mulStab) * (s ∩ a • C.mulStab * (t ∩ a • C.mulStab)).mulStab)).card ≤
    ↑(((s ∪ t) * C.mulStab) \ ((s ∪ t) * (s ∩ a • C.mulStab * (t ∩ a • C.mulStab)).mulStab)).card