anchor.diff:  
       override this.ToString() = this.charset  
   
  -    static member Create (charset:string) =
  +    static member Create (charset:string) = 
           match charset.ToUpperInvariant() |> CharsetHelper.Charsets.TryFind  with
           | Some charset -> charset
           | None -> 
               match parse CharsetHelper.Parser charset with 
               | Success (charset, _) -> charset
  +
               | _ -> invalidArg "charset" "not a charset"
   
       static member ISO_8859_1 = CharsetHelper.ISO_8859_1
  
positive.code:     static member UTF_8 = CharsetHelper.UTF_8 
  
  and [<AbstractClass; Sealed;>] internal CharsetHelper () = 
      static member val ISO_8859_1 = Charset.Create "ISO-8859-1"
  
      static member val US_ASCII  = Charset.Create "US-ASCII"
  
negative.code:     static member val UTF_16LE = Charset.Create "UTF-16LE" 
  
      static member val UTF_8 = Charset.Create "UTF-8" 
  
  [<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>]
  module internal Charset = 
  