∀ {s : String.Slice} (p₁ : s.Pos) {p₂ : s.Pos} (h : p₁ ≤ p₂) (l : String.Slice.Pattern.Model.SlicesFrom p₂),
  String.Slice.Pattern.Model.SlicesFrom.extend p₁ h l =
    {
      l :=
        match l.l, ⋯ with
        | st :: sts, h_1 => st.extendLeft p₁ ⋯ :: sts,
      any_head? := ⋯ }