TODO list for Perl module DateTime-Event-ICal

* high frequency FREQ can be optimized by precomputing larger steps of INTERVAL

* include this in docs:
  http://www.w3.org/2001/sw/Europe/200210/calendar/vocab_usecases.html
  http://www.imc.org/ietf-calendar/mail-archive/msg00623.html

* optimize yealy/monthly + interval + byday
  - uses intersection with "all days"

* "byday -2 friday"
  - test overflow
  - need some 'interval' and 'dtstart' tests

* the implementation of 'select' in Set::Infinite is incomplete
  and doesn't have the 'last' attribute. -- should be implemented!
  Note: this is not a problem because it is only used in 'count', 
        which generates finite sets. 

* finish importing Date::Set RFC2445 recurrence tests

* bysetpos
  - use some kind of memoization

* byweekno - test with 'daily'

* no tests were made defining recurrences without dtstart.

* the effect of 'count' on a recurrence without dtstart is not defined.
  the effect of count == 0 is not defined.

* no tests were made using datetimes with timezones.

* mixing positive and negative arguments in the same list
  might give wrong results is the numbers are big enough
  to cause the datetimes to interleave.
  - split in two recurrences and join with 'union'

* some arguments are not checked for overflow,
  such as byyearday.