public static bool TryParse(Queue <Token> unProcessedTokens, out TzDbRule?tzDbRule)
        {
            // the value of the first token of every link entry is Rule
            const String ruleRegex = "Rule";

            if (Regex.IsMatch(unProcessedTokens.Peek().Value, ruleRegex))
            {
                try
                {
                    // dequeue Rule token and discard
                    unProcessedTokens.Dequeue();

                    string name = unProcessedTokens.Dequeue().Value;
                    AbstractYearSpecification from = YearSpecificationExpression.Parse(unProcessedTokens);
                    AbstractYearSpecification to   = ParseToExpression(unProcessedTokens, from);
                    string      type = unProcessedTokens.Dequeue().Value;
                    MonthOfYear @in  = MonthOfYearExpression.Parse(unProcessedTokens);
                    AbstractDayOfMonthSpecification on = DayOfMonthSpecificationExpression.Parse(unProcessedTokens);
                    AbstractTime at   = TzDbTimeExpression.Parse(unProcessedTokens);
                    TimeSpan     save = TimeSpanExpression.Parse(unProcessedTokens);
                    TimeZoneAbbreviationVariable letter = TimeZoneAbbreviationVariableExpression.Parse(unProcessedTokens);

                    tzDbRule = new TzDbRule(name, from, to, type, @in, on, at, save, letter);

                    Token endLineToken = unProcessedTokens.Dequeue();
                    // guard end of line token was present
                    if (endLineToken.TokenType != TokenType.EndLine)
                    {
                        string msg = String.Format(
                            "received unexpected token type:{0} when token type should have been {1}",
                            endLineToken.TokenType, TokenType.EndLine);
                        throw new Exception(msg);
                    }

                    return(true);
                }
                catch (Exception e)
                {
                    string msg = String.Format("Unable to parse {0}", typeof(RuleExpression).Name);

                    throw new Exception(msg, e);
                }
            }
            tzDbRule = null;
            return(false);
        }
        public static bool TryParse(Queue <Token> unProcessedTokens, out DayOfYearSpecification?dayOfYearSpecification)
        {
            MonthOfYear?monthOfYear;

            if (MonthOfYearExpression.TryParse(unProcessedTokens, out monthOfYear))
            {
                AbstractDayOfMonthSpecification dayOfMonthSpecification;
                if (!DayOfMonthSpecificationExpression.TryParse(unProcessedTokens, out dayOfMonthSpecification))
                {
                    // default is first day of the month
                    dayOfMonthSpecification = new ExplicitDayOfMonthSpecification(1);
                }

                dayOfYearSpecification = new DayOfYearSpecification(monthOfYear.Value, dayOfMonthSpecification);
                return(true);
            }

            dayOfYearSpecification = null;
            return(false);
        }