∀ {α₁ : Type u} {α₂ : Type v} {β₁ : Type w} {β₂ : Type x} {f₁ : Filter α₁} {f₂ : Filter α₂} {m₁ : α₁ → β₁}
  {m₂ : α₂ → β₂}, Filter.map m₁ f₁ ×ˢ Filter.map m₂ f₂ = Filter.map (fun p => (m₁ p.1, m₂ p.2)) (f₁ ×ˢ f₂)