DHCPD classes - How to put unknown devices in a separate class?

[Jessica Meyer]

Hi list

I'm stuck with DHCPD classes. I need to classify all requests
according to the vendor-class-identifier. I have two classes and it
works fine. Now what I need (and haven't found yet) is to put all
_other_ devices in a third class. So, if a device doesn't belong to
class A or B, put it in C. I have played with "match if" statements
but haven't had any success.

My classes are:

class "Devices_A" {
    match if ( substring ( option vendor-class-identifier, 0, 6 ) = "vendor1" );
    log ( info, "Got a packet from a vendor1 device" );
}

class "Devices_B" {
    match if ( substring ( option vendor-class-identifier, 0, 6 ) = "vendor2" );
    log ( info, "Got a packet from a vendor2 device" );
}

Can i somehow test if a request doesn't belong to Devices_A or
Devices_B? Any help is highly appreciated!
Jess