mapped-devices documentation question (multiple devices)

[ng0 <contact.ng0@cryptolab.net>]

Hi,

upon trying to create a system with multiple mapped-devices the
current documentation about mapped-devices doesn't help me anymore.

So far I'm just trying with two luks devices, I'll get RAID in there as
a third device later.
So this part in a config.scm:

  (mapped-devices
   (list (mapped-device
          (source (uuid "fooooo1233333333333333333"))
          (target "my-root")
          (type luks-device-mapping))
         (mapped-device
           (source (uuid "fooooo124444444444444444"))
           (target "dg2")
           (type luks-device-mapping))))

  (file-systems (cons* (file-system
                        (device "my-root")
                        (title 'label)
                        (mount-point "/")
                        (type "ext4")
                        (dependencies mapped-devices))
                      (file-system
                        (device "/dev/sdb1")
                        (title 'device)
                        (mount-point "/mnt/dg1")
                        (type "ext4"))
                      (file-system
                        (device "dg2")
                        (title 'label)
                        (mount-point "/mnt/dg2")
                        (type "ext4")
                        (dependencies mapped-devices))
                      %base-file-systems))

...fails with this error:

user@greendragon ~$ guix system build /etc/config.scm                                                                              
guix system: error: service 'file-system-/mnt/dg2' requires
'device-mapping-my-root', which is not provided by any service

How do I fix this?