[Exim] string expansion and smtp_accept_max_per_host

Hi all. I've been trying for hours to get my smtp_accept_max_per_host
working right, but I have failed and am now asking for help.

I've tried many many configurations, including the one in the FAQ
(Q9812):

smtp_accept_max_per_host = \
     ${lookup{$sender_host_address}lsearch{/opt/exim/lib/smtp.connections}\
     {$value}\
     {\
     ${lookup{${mask:$sender_host_address/24}lsearch*{/opt/exim/lib/smtp.connect
ions}}\
     }}

This results in a
expansion of smtp_accept_max_per_host failed for host...missing lookup type
each time a host connects.

It's also not completely clear to me how I would add lines for /16 and /8
lookups.

My smtp.connections file is:
127.0.0.1:      0
198.15.157.0/24:0
192.168.0.0/16: 0
*:3

Ideally, I'd like to use a hostlist instead of a file lookup. Can I do
something like

hostlist smtp_unlimited_hosts = 127.0.0.1/32 : \
    198.15.157.0/24 : \
    192.168.0.0/16 :

smtp_accept_max_per_host = \
        if ("${lookup{$sender_host_address} +smtp_unlimited_hosts {$value}}" is
 "0") \
        else {$value} is "3" \
        endif

I tried variations on this theme, but couldn't get any to work.

Thank you...