∀ {ρ : Type} {pat : ρ} [inst : String.Slice.Pattern.Model.PatternModel pat] {s : String.Slice} {pos₁ pos₂ : s.Pos},
  String.Slice.Pattern.Model.IsLongestMatchAt pat pos₁ pos₂ =
    ∃ (h : pos₁ ≤ pos₂), String.Slice.Pattern.Model.IsLongestMatch pat (pos₁.sliceFrom pos₂ h)