{s : String} →
  {off : String.Pos.Raw} →
    {motive : String.Pos.Raw.IsValid s off → Sort u} →
      ((le_rawEndPos : off ≤ s.rawEndPos) →
          (isValidUTF8_extract_zero : (s.toByteArray.extract 0 off.byteIdx).IsValidUTF8) → motive ⋯) →
        (t : String.Pos.Raw.IsValid s off) → motive t