no goals
ι : Type ?u.330793
α : Type u
β : Type v
γ : Type w
δ : Type x
l₁ l₂ : List α
p : α → Prop
f : (a : α) → p a → β
l : List α
H : ∀ (a : α), a ∈ l → p a
⊢ pmap f l H = [] ↔ l = []
rw [← length_eq_zero, length_pmap, length_eq_zero]