Conditional expression inside pool

[Sharkey, Patrick M.]

What is the correct syntax for conditionally setting the next-server and
filename directives inside a pool?

I have one subnet defined where I permit unknown clients to obtain a
lease. This is to accommodate systems performing a PXE boot. However, I
discovered that known clients were unable to obtain a lease when PXE
booting, so I added the next-server and filename directives to the known
clients pool. I would prefer to conditionally set these directives if
the client is a member of class "PXE" as defined below:

class "PXE" {
        match if substring (option vendor-class-identifier, 0, 9) =
"PXEClient";
}

As it stands, all known clients receive the next-server and filename
options in the response (regardless of whether they PXE boot or not).
Although this does not appear to cause a problem, it is not ideal.

subnet 192.168.152.0 netmask 255.255.254.0 {
        option routers 192.168.153.254;

        # Unknown clients get this pool.
        pool {
                allow members of "PXE";
                deny dynamic bootp clients;
                max-lease-time 7200;
                range 192.168.153.60 192.168.153.109;
                allow unknown-clients;
                next-server 192.168.153.26;
                filename "OSChooser\\i386\\startrom.com";
       }

        # Known clients get this pool.
        pool {
                deny dynamic bootp clients;
                max-lease-time 28800;
                range 192.168.153.110 192.168.153.199;
                deny unknown-clients;
                next-server 192.168.153.26;
                filename "OSChooser\\i386\\startrom.com";
        }
}

What I would like to know is how to wrap the following directives in a
conditional (member of class "PXE"):

next-server 192.168.153.26;
filename "OSChooser\\i386\\startrom.com";


Thanks in advance.