[exim] $acl_m0 in logwrite?

hello,

for testing of the variable $acl_m0, i will write this value in main
logfile.
But nothing is done with the following command:

warn logwrite = :main: ...acl_m0 $acl_m0

how can i test the value of acl_m0?
i will use this value in contentscanning condition for an sql-lookup.

thank for help

maik