simp
case inl
α : Type u_1
inst✝¹ : CommGroup α
inst✝ : DecidableEq α
s✝ s' t✝ t' C : Finset α
a✝ b a : α
s t : Finset α
has : a ∈ s
hst : (s ∩ a • ∅.mulStab * (t ∩ a • ∅.mulStab)).mulStab ⊆ ∅.mulStab
⊢ ↑∅.mulStab.card - ↑(s ∩ a • ∅.mulStab * (s ∩ a • ∅.mulStab * (t ∩ a • ∅.mulStab)).mulStab).card -
      ↑(t ∩ a • ∅.mulStab * (s ∩ a • ∅.mulStab * (t ∩ a • ∅.mulStab)).mulStab).card ≤
    ↑((s ∪ t) * ∅.mulStab).card - ↑((s ∪ t) * (s ∩ a • ∅.mulStab * (t ∩ a • ∅.mulStab)).mulStab).card
no goals