anchor.diff:  check_options([]) ->
       ok.
   
  +is_option_valid({default_tags, List}) when is_list(List) ->
  +    %% This fun() returns true for any invalid entries, to keep only
  +    %% those.
  +    Fun = fun
  +        ({{non_specific, A}, B}) ->
  +            not (io_lib:char_list(A) andalso io_lib:char_list(B));
  +        ({A, B}) ->
  +            not (io_lib:char_list(A) andalso io_lib:char_list(B));
  +        (_) ->
  +            true
  +    end,
  +    case lists:filter(Fun, List) of
  +        [] -> true;
  +        _  -> false
  +    end;
   is_option_valid({doc_version, {Major, Minor}}) when
     is_integer(Major) andalso Major >= 0 andalso
     is_integer(Minor) andalso Minor >= 0 ->
  
positive.code:     %% Non-specific tags are associated to nodes which don't have an
      %% explicit tag or to those with the "!" explicit non-specific tag.
      %% The non-specific tags are resolved using a schema.
      %% TODO.
      Tags3 = dict:store({default, {non_specific, "!"}}, "tag:yaml.org,2002:",
        Tags2),
      Tags4 = dict:store({default, {non_specific, "?"}}, "tag:yaml.org,2002:",
        Tags3),
      Parser#yaml_parser{
        tags = Tags4
      }.
  
  %% -------------------------------------------------------------------
  
negative.code:       extra = [{option, Option}]
      },
      Error1 = case Option of
          {default_tags, _} ->
              Error#yaml_parser_error{
                text = "Invalid value for option \"default_tags\": "
                "it must be a list of {Prefix, Prefix_Value}.\n"
              };
          {doc_version, _} ->
              Error#yaml_parser_error{
                text = "Invalid value for option \"doc_version\": "
  