[exim] acl filter variable syntax

Hello,
I'm having trouble getting the syntax correct for checking the values of some header lines.

I would like a condition to be true where a certain header line exists, and, if another header line contains a keyword from file.

Example:

condition = ${if def:header_X-FC-System:} and ${if header_Return_Path: contains {/etc/exim4/listofnames}}

The email header X-FC-System has to exist, and the Return_Path header needs to have a value of a line
from the listofnames file.

Any help making this syntax proper?

Thanks in advance!

Cheers,
Mike