1 * do not stop on first old item, consume whole feed
2 channel pubDate is not old.
3 * check whether new items are retrieved, but old ones are
5 * item overwrite only if date >= olddate
6 else notice, skip offending item, and do not change
7 feed date, continue but exit with error
8 (invariant: date of every unfetched item is > feed date)
9 * Make directories/items read-only.
10 * check whether dates are handled properly (timezones!)
11 * fix remaining bugs (see README)
12 * support lastBuildDate and pubDate for channels
13 * replace assertions by proper error handling
14 * clean state after a tag level is left, so we can pipe
15 multiple streams, one after another, into one instance.