[Exim] combined conditional statements

Can anyone who understands Exim help me with the following condition I
am trying to set up:

condition = ${if or  { ${lookup dnsdb
{mx=$sender_address_domain}{1}{0}} } { ${lookup dnsdb {a=
$sender_address_domain}{1}{0}} }      }

What I am not looking for is verify = sender. I have tried murderous
amounts of variations on the above condition, and have used exim -bh
extensively during my glorious failures. I can get one or the other to
work, but not both (being mx or a).

Thanks,
Robert