The signatures feature has been "experimental" since Perl 5.20 (May 2014), but
expected to stay. After 8 years I'm ready to take the risk :)
Have added Perl v5.28 (June 2018) as the minimum requirement, even though the
current revbank should work with 5.20, to see if this bothers any users. Perl
v5.28 is in Debian "buster", which is now oldstable.
The 'stock' plugin uses the 'product_id' field in the cart item, but
this was not copied by the 'repeat' plugin. The confusion is caused by a
mismatch between the API for RevBank::Cart->add and the internal
representation.
