anchor.diff:                  match UnicodeCharSet.ofAbbreviation abbrev with
                   | None ->
                       return
  -                        [ sprintf "Unknown or invalid Unicode category specified. (Category = %s)" abbrev ]
  +                        sprintf "Unknown or invalid Unicode category specified. (Category = %s)" abbrev
  +                        |> LazyList.singleton
                           |> Choice2Of2
                   | Some categoryCharSet ->
                       return
  
positive.code:             match atLeast, atMost with
              | None, None ->
                  return
                      ["Invalid number of repetitions. Either the minimum or maximum (or both) number of repetitions must be specified."]
                      |> Choice2Of2
  
              | None, Some atMost ->
  
negative.code:                         |> Choice1Of2
              else
                  return
                      "Unicode category patterns may not be used unless the 'Unicode' compiler option is set."
                      |> LazyList.singleton
                      |> Choice2Of2
  
          | Pattern.Negate r ->
  