rewrite [List.countP_eq_length_filter]
α : Type u
β : Type v
inst✝³ : BishopFin α
inst✝² : BishopFin β
p q : α → Prop
inst✝¹ : DecidablePred p
inst✝ : DecidablePred q
⊢ List.countP (fun a => decide (p a)) (elems α) = card { a // p a }
α : Type u
β : Type v
inst✝³ : BishopFin α
inst✝² : BishopFin β
p q : α → Prop
inst✝¹ : DecidablePred p
inst✝ : DecidablePred q
⊢ List.length (List.filter (fun a => decide (p a)) (elems α)) = card { a // p a }