[Exim] Router condition lookup: line-delimited file contains

Hi folks,

I'm lost in lookups within router conditions.

Is there some foo I can do in a router condition that resolves to true
if a line-delimited file contains some string? Something like

condition = ${lookup{${local_part}@${domain}}XXX{/path/to/addrfile}{1}}

where addrfile contains a line-delimited list of email addresses.

I've spent an hour in the Exim spec and can't seem to find a lookup that
just tests for the presence of a key in a file. The best I can do is
suck the entire file into a string with the readfile expansion operator
and then match against that.

Thanks,
Sheldon.