[exim] ACL Question - using acl_cX or checking for headers?

Hi,

I'm trying to mark mails which has no reverse Lookup and are listed in
an rbl.

My acls look like this:

warn    message  = X-DNSbl-Warning: $sender_host_address is listed in
$dnslist_domain
            set acl_c1 = DNSbl-Warning
            dnslists = list.dsbl.org : \
                           dnsbl.sorbs.net : \
                           dnsbl.njabl.org : \
                           bl.spamcop.net : \
                           dsn.rfc-ignorant.org : \
                           sbl-xbl.spamhaus.org : \
                           l1.spews.dnsbl.sorbs.net

            warn    message = X-rDNS: $sender_host_address has no rDNS
                        set acl_c2 = rDNS
                        !verify     = reverse_host_lookup

            warn    message = mailserver has no reverse lookup and is
listed in a blacklist
                        condition = ${if and { {eq
{$acl_c1}{DNSbl-Warning}}{eq {$acl_c2}{rDNS}} } {yes}{no} }

My experience shows me that acl_c1 and c2 are alway set. So my last
condition is always true. First I tried to check if the two headers
X-DNSbl-Warning and X-rDNS are present but the condition I created
always fail...

Is there a way to set the acl_cX only if the header is also set or how
should a condition looks like for checking the headers? I had something
like:

            condition = ${if  and
{{def:h_X-DNSbl-Warning:}{def:h_X-rDNS}} {yes}{no}   }

but this did not work either.

thank you
torben