anchor.diff:      ;   {ok, Value} ->
               case of_bin(Value)
               of  {error, {data_format_invalid, _}=ParsingError} ->
  +                    {error, {internal, ParsingError}}
  +            ;   {error, {field_missing, _}=ParsingError} ->
                       {error, {internal, ParsingError}}
               ;   {ok, #t{expiry=ExpiryOpt}=T} ->
                       ExpiryToOkOrError =
  
positive.code:     }.
  
  -type parsing_error() ::
      {data_format_invalid, binary()}.
  
  -type retrival_error() ::
        {internal, parsing_error()}
  
negative.code: -spec of_props(t_props()) ->
      hope_result:t(t(credentials_type()), parsing_error()).
  of_props(Props) ->
      % TODO: Check for all missing fields, not just the first.
      MakeFieldGetter =
          fun (Key) ->
              fun (Acc) ->
  