∀ {α : Type} {n : ℕ} {p : α → Bool} {b : α} {xs : Vector α n},
  Vector.find? p xs = some b ↔
    p b = true ∧
      ∃ k₁ k₂, ∃ (w : n = k₁ + 1 + k₂), ∃ as bs, xs = Vector.cast ⋯ (as.push b ++ bs) ∧ ∀ a ∈ as, (!p a) = true