∀ {ρ : Type} {pat : ρ} [inst : String.Slice.Pattern.Model.PatternModel pat] {s : String.Slice} {p : s.Pos}
  {l : List (String.Slice.Pattern.SearchStep s)},
  p = s.startPos → l = [] → String.Slice.Pattern.Model.IsValidRevSearchFrom pat p l