intro x hx
α✝¹ : Type u_1
α✝ : Type u_2
f : α✝¹ → Option α✝
xs : List α✝¹
h : List.filterMap f xs = []
⊢ ∀ (x : α✝¹), x ∈ xs → f x = none
α✝¹ : Type u_1
α✝ : Type u_2
f : α✝¹ → Option α✝
xs : List α✝¹
h : List.filterMap f xs = []
x : α✝¹
hx : x ∈ xs
⊢ f x = none