∀ {α : Type u_1} {p : α → Option α} {l : List α} {a a' : α},
  a ∈ l →
    p a = some a' →
      ∃ a a' l₁ l₂, (∀ b ∈ l₁, p b = none) ∧ p a = some a' ∧ l = l₁ ++ a :: l₂ ∧ List.replaceF p l = l₁ ++ a' :: l₂