Retention policy per year

Hi, as far as I understand section RETENTION POLICY from man btrbk.conf, it is currently not possible to define a count per year or per any other period.

For instance, I currently have:
`target_preserve 30d 24w 12m 8y`
That means 30 days, 24 weeks, 12 months and 8 years are kept. So after 12 months, only 1 snapshot per year is kept. That means I can't define that after 12 months I like to keep 2 snapshots per year.

A proposal how to define such a policy: Additionaly to the current keywords
`[h] [d] [w] [m] [y]`
add the new keywords
`[ph] [pd] [pw] [pm] [py]`

For my case the new policy would be:
`target_preserve 30d 24w 12m 8y 2py`

Please comment and rate this idea...