∀ {ι : Type u_1} {π : ι → Sort u_2} {s t : Finset ι} [inst : (i : ι) → Decidable (i ∈ s)]
  [inst_1 : (i : ι) → Decidable (i ∈ t)],
  t ⊆ s → ∀ (f g₁ g₂ : (a : ι) → π a), s.piecewise f (t.piecewise g₁ g₂) = s.piecewise f g₂