anchor.diff:                  | [] -> List.rev acc // TODO: refactor code to remove this
                   | c :: t when Char.IsLetterOrDigit c |> not ->
                       let offset = getOffset t
  -                    failwith (sprintf "Invalid css selector syntax (char '%c' at offset %d)" c offset)
  +                    []
  +                    //failwith (sprintf "Invalid css selector syntax (char '%c' at offset %d)" c offset)
                   | _ ->
  -                    failwith "Invalid css selector syntax"
  +                    //failwith "Invalid css selector syntax"
  +                    []
               tokenize' [] source
   
       type FilterLevel = 
  
positive.code:                 | [] -> 
                      acc, []
                  | _ ->
                      failwith "Invalid css selector syntax"
          
              let (|TokenStr|_|) (s:string) x  =
                  let chars = s.ToCharArray() |> Seq.toList
  
negative.code:     let (?) (node : HtmlNode) name = 
          HtmlNode.attributeValue name node 
  
  module CssSelectors =
      type SelectorToken =
          | ClassPrefix of int
          | IdPrefix of int
  