Guix and GuixSD

I've been trying to begin a transition from NixOS to Guix, but I'm having a hard time finding where the different symbols referenced in the Guix/Guile manual are defined. For example, I want to use BTRFS on a LUKS-encrypted device, but I can't quite find what to #:use-module for the mapped-devices symbol. That is to say:

(file-system
  (device "/dev/mapper/cryptroot")
  (mount-point "/home")
  (type "btrfs")
  (options "subvol=home")
  (dependencies mapped-devices))

The mapped-devices above is an "unbound variable" (according to the errors). I still don't have this figured out, so how would others go about finding what to "import" since the official examples I'm looking at don't really tell me anything? (unless I'm blind, very likely!)