TODO list for DT::Set, DT::Span, and DT::SpanSet

- DT::Set needs a fix to avoid generating _huge_ in-memory sets,
  when the result of an operation is a finite set.
  For example: "every minute in year 2003"
  (see SKIP in DateTime::Incomplete t/02incomplete.t)

- implement as_list() in DT::SpanSet.pm

- implement previous() in DT::SpanSet.pm

- discuss functions like 
    is_infinite
    is_empty 
    cardinality ?

- implement faster DT::Set::complement(), 
  using 'function composition'

- move all 'set-specific' code to Set::Infinite::_recurrence 

- functions like before(), after(), during() might be useful
  in DT::Span

- tests for DT::Span and DT::SpanSet 'duration()'
- duration() returns 'Inf' scalar - should be a DT::Duration::Infinite

