Fancy splitting into non-nnml backend

[Arcady Genkin]

Greetings:

I can't set up nnml-split-fancy to work with nnmaildir backend (i.e.
save the split messages into nnmaildir groups).  It seems as though it
is not possible to specify the backend in the group name: if I pass
"nnmaildir:foo" as the group name, the messages get saved into an nnml
group (not nnmaildir) called "nnmaildir:foo".

Can nnmail-split-fancy split into anything other than nnml backend?

I currently have gnus-secondary-select-methods specifying three
servers: nnmaildir, nnml and nnimap.  (I am trying to migrate all nnml
groups to nnmaildir).  Then, there is the following configuration:

(defun arcady-dated-mailbox (prefix)
  (concat "nnmaildir:"
          prefix
          "-"
          (format-time-string "%Y-%m")))

(setq
 nnmail-split-methods 'nnmail-split-fancy
 nnmail-split-fancy
      '(|
        ("X-CDF-Spam-Status-Bogofilter" "\\bSpam\\b"
         (: arcady-dated-mailbox "spam"))
        ("X-CDF-Spam-Status-Bogofilter" "\\bUnsure\\b"
         (: arcady-dated-mailbox "spam-unsure"))
        (: arcady-dated-mailbox "cdf")))

Many thanks in advance,
-- 
Arcady Genkin