intro hget
case mpr
α : Type u_1
β : Type u_2
i : Nat
xs : List α
bs : List Bool
pf1 : i < length xs
pf2 : i < length bs
f : α → β
hu : Unique (map f xs)
⊢ get bs { val := i, isLt := pf2 } = true →
    get (map f xs) { val := i, isLt := (_ : i < length (map f xs)) } ∈ map f (sieve bs xs)
case mpr
α : Type u_1
β : Type u_2
i : Nat
xs : List α
bs : List Bool
pf1 : i < length xs
pf2 : i < length bs
f : α → β
hu : Unique (map f xs)
hget : get bs { val := i, isLt := pf2 } = true
⊢ get (map f xs) { val := i, isLt := (_ : i < length (map f xs)) } ∈ map f (sieve bs xs)