(some foo._unfold)
foo._unfold : ∀ (xs ys zs : List Nat),
  foo xs ys zs =
    match (xs, ys) with
    | (xs', ys') =>
      match zs with
      | z :: zs => foo xs ys zs
      | x =>
        match ys' with
        | [] => [1]
        | x => [2]