- new should only be used to make empty sets (?)

- DateTime::SpanSet->from_recurrences

- finish DateTime::SpanSet->from_sets 

- finish DateTime::SpanSet->from_set_and_duration

- discuss functions like 
    is_infinite
    is_empty 
    start_is_close 
    end_is_open

    cardinality ?

- More tests for DateTime::Span

- Tests for DateTime::SpanSet

- New constructors-from-duration API:

    $set1 = DateTime::SpanSet->new(
        dates => [ $dt_span, $dt_span ],
        duration => $dt_duration );     

