∀ {ι : Type u_1} {μ : Type u_2} [inst : DecidableEq ι] [inst_1 : AddCommMonoid μ] [inst_2 : Finset.HasAntidiagonal μ]
  [inst_3 : DecidableEq μ] (s : Finset ι) (n : μ),
  s.finsuppAntidiag n =
    Finset.map { toFun := fun f => { support := {x ∈ s | ↑f x ≠ 0}, toFun := ↑f, mem_support_toFun := ⋯ }, inj' := ⋯ }
      (s.piAntidiag n).attach