Hello, Guix developers! There no suggestions in manual about how to delete system generations that are displaying by `guix system list-generations`

I do not use them, just like to update))

# guix system list-generations | grep Generation
Generation 1    Sep 17 2018 18:23:13
Generation 2    Nov 13 2018 20:19:47
Generation 3    Nov 14 2018 06:50:53
Generation 4    Nov 27 2018 21:04:19
Generation 5    Nov 28 2018 09:42:41
Generation 6    Nov 28 2018 09:45:06
Generation 7    Nov 28 2018 09:45:06
Generation 8    Nov 28 2018 09:53:49
Generation 9    Nov 28 2018 09:53:49
Generation 10    Nov 28 2018 09:53:49
Generation 11    Nov 28 2018 09:53:49
Generation 12    Nov 28 2018 13:41:47
Generation 13    Nov 28 2018 09:53:49
Generation 14    Nov 29 2018 09:50:51
Generation 15    Nov 30 2018 18:04:51
Generation 16    Nov 30 2018 18:04:51
Generation 17    Nov 30 2018 18:04:51
Generation 18    Nov 30 2018 18:04:51
Generation 19    Dec 02 2018 16:12:50
Generation 20    Dec 02 2018 18:24:11
Generation 21    Dec 02 2018 18:24:11    (current)

# guix gc

...

note: currently hard linking saves 15409.75 MiB
guix gc: freed 145.6875 MiBs

How to delete system generations?