Enumerating occurrences hangs..

Building an RiCal.Event as follows:

=> BEGIN:VEVENT DTSTART;VALUE=DATE-TIME:20130805T153159Z
SUMMARY:Summary
RRULE:FREQ=MONTHLY
RRULE:FREQ=HOURLY
EXRULE:FREQ=HOURLY
END:VEVENT

The net result of this should be that the HOURLY EXRULE cancels out the HOURLY RRULE. It should generate monthly recurrences.

Per RFC 2445, page 112:
Multiple instances of the "RRULE" and
"EXRULE" properties can also be specified to define more sophisticated recurrence sets. The final recurrence set is generated by gathering all of the start date-times generated by any of the specified "RRULE" and "RDATE" properties, and then excluding any start date and times which fall within the union of start date and times generated by any specified "EXRULE" and "EXDATE" properties.

When I run the following, it hangs forever & pegs the CPU (I include CTRL-C):

1.9.3p286 :085 > ev.occurrences(:count => 10)
^CIRB::Abort: abort then interrupt!
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/fast_date_time.rb:74:in `call'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/fast_date_time.rb:74:in `<=>'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/property_value/date_time.rb:190:in `cmp_fast_date_time_value'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/property_value/date_time.rb:186:in `<=>'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:86:in `<'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:86:in `exclusion_for'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:110:in `next_occurrence'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:137:in `each'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:173:in `to_a'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:173:in `to_a'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:193:in `occurrences'
    from (irb):85
    from /home/paul/.rvm/rubies/ruby-1.9.3-p286/bin/irb:16:in `<main>'

In my own testing, I found that removing the HOURLY EXRULE solves this. I found I can add DAILY or MONTHLY RRULES & corresponding EXRULE's, and these work fine. But in my own testing, any HOURLY EXRULE hangs a computation of occurrences.

Here is the console commands I used to produce:

1.9.3p286 :124 >   ev = RiCal.Event
1.9.3p286 :124 >   ev.summary = 'Summary'
1.9.3p286 :124 >   ev.add_rrule({:freq => 'monthly'})
1.9.3p286 :124 >   ev.add_rrule({:freq => 'hourly'})
1.9.3p286 :124 >   ev.add_exrule({:freq => 'hourly'}
1.9.3p286 :124 >   ev.dtstart = DateTime.now
1.9.3p286 :122 > ev.occurrences(:count => 1)
^CIRB::Abort: abort then interrupt!
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/fast_date_time.rb:73:in `call'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/fast_date_time.rb:73:in `<=>'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/property_value/date_time.rb:190:in `cmp_fast_date_time_value'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/property_value/date_time.rb:186:in `<=>'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/property_value/recurrence_rule/occurrence_incrementer.rb:101:in `update_cycle_range'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/property_value/recurrence_rule/occurrence_incrementer/frequency_incrementer.rb:64:in `next_cycle'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/property_value/recurrence_rule/occurrence_incrementer.rb:90:in `next_time'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/property_value/recurrence_rule/enumerator.rb:69:in `next_occurrence'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:62:in `block in next_occurrence'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:62:in `each'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:62:in `each_with_index'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:62:in `next_occurrence'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:109:in `next_occurrence'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:137:in `each'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:173:in `to_a'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:173:in `to_a'
    from /home/paul/.rvm/gems/ruby-1.9.3-p286/gems/ri_cal-0.8.8/lib/ri_cal/occurrence_enumerator.rb:193:in `occurrences'
    from (irb):122
1.9.3p286 :124 >   ev
 => BEGIN:VEVENT
DTSTART;VALUE=DATE-TIME:20130805T153159Z
SUMMARY:Summary
RRULE:FREQ=MONTHLY
RRULE:FREQ=HOURLY
EXRULE:FREQ=HOURLY
END:VEVENT

Thanks.

EDIT: I found similar behavior when adding SECONDLY, MINUTELY, or DAILY RRULE's and EXRULE's (with an existing MONTHLY RRULE). WEEKLY RRULE & EXRULE did not hang however.