∀ {ρ : Type} (pat : ρ) (s : String.Slice) [inst : String.Slice.Pattern.BackwardPattern pat]
  [String.Slice.Pattern.StrictBackwardPattern pat] {it it' : Std.IterM Id (String.Slice.Pattern.SearchStep s)},
  it'.IsPlausibleSuccessorOf it → InvImage WellFoundedRelation.rel (fun it => it.internalState.currPos.down) it' it