- remove negative-duration hacks - check if there are bugs in DT::Duration!

- intersection() function composition in DateTime::Set
    - check 'span'
    - add tests
    - 'union' too ?

- check 'span' in next/previous methods
    - add tests

- as_list in SpanSet.pm
- previous in SpanSet for iterators

- discuss functions like 
    is_infinite
    is_empty 

    cardinality ?

