Update TODO
Eliminate some magic numbers.
Use tags to mark the properties of the patterns.
Part I of matching trailing IPv4 addresses.

This step implements matches it only for addresses without contractions.
Add (?k<unit>:)(?k<unit>:) to pattern.

If (?|) is used, it's important that each alternation contains the
same number of named captures. That this is necessary can be considered
a bug in Perl.
Testing patterns that have "-trailing_ipv4 => 1".
We may need an empty trailing subpattern.