[Exim] confusion with 4.12 and local_part_prefix

Hi,

Under 3.3x I had the following transport setup

#
# Deliver to LMTP for Cyrus mailboxes
#
local_lmtp:
        driver = smtp
        protocol = lmtp
        batch_max = 1
        hosts = 127.0.0.1
        allow_localhost

Which had the following director:

#
# strip cyrus- off the front and then deliver over lmtp to the cyrus
# system
#

cyrus_users:
        driver = smartuser
        transport = local_lmtp
        domains = cyrus.example.com
        prefix = "cyrus-"

Mail addressed to cyrus-<username>@cyrus.example.com got dumped into
the mailbox for user <username> in the cyrus system over TCP LMTP.

The converter from 3.3x to 4.1x config files rewrote this as:

#
# Deliver to LMTP for Cyrus mailboxes
#
local_lmtp:
#!!# batch_max renamed connection_max_messages
driver = smtp
allow_localhost
connection_max_messages = 1
hosts = 127.0.0.1
protocol = lmtp
user = exim

and the director (oops, router) as:

#
# strip cyrus- off the front and then deliver over lmtp to the cyrus
# system
#

cyrus_users:
#!!# prefix renamed local_part_prefix
driver = accept
domains = cyrus.example.com
local_part_prefix = "cyrus-"
retry_use_local_part
transport = local_lmtp


Ok so far. Except that the exim 4.1x combo tries to deliver to
cyrus-username@??? over LMTP, which is rejected by the
cyrus daemon (the cyrus- prefix should have been stripped). Am I
misunderstanding the 4.x manual? My reading of it says the above
should work.

Thanks,

Gary