How to configure retention periods for non-standard schedules?

[davidfraser]

How can retention periods via keep-within be set for schedules that aren't hourly, daily, weekly etc?

I have multiple schedules set for the same profile in profiles.yaml:

  backup:
    schedule:
      - hourly
      - "*-*-* 0,4,8,12,16,20:00:00"
  retention:
    keep-within-hourly: "7d"
    keep-within-daily: "30d"

But I can't see how to set a longer retention period for the four-hourly backup. I think the restic source code is here and from a brief look it seems that it's doing some kind of comparison to the duration rather than an exact match, but that would mean only something longer than a day would get a different schedule...

Is this possible?

PS I am appreciating resticprofile's ability to set up multiple schedules, especially with the mixins in the v2 config