∀ {ι : Type u_1} {κ : Type u_2} {M : Type u_3} [inst : CommMonoid M] {f : ι → M} [inst_1 : DecidableEq ι] {s : Finset κ}
  {t : κ → Finset ι} (a : ι), (a ∈ s.biUnion fun k => {i ∈ t k | f i ≠ 1}) ↔ a ∈ {i ∈ s.biUnion t | ¬f i = 1}