∀ {α : Type u_1} {β : Type u_2} {M : Type u_3} [inst : Zero M] (f : α →₀ β →₀ M) (a : α × β),
  a ∈ f.support.disjiUnion (fun a => Finset.map (Function.Embedding.sectR a β) (f a).support) ⋯ ↔ (f a.1) a.2 ≠ 0