How Use XSLT to Test For Multiple Values

Hi,
I have an xml file with zero, one or more tags something like this:

                                        <DeviceCode>E</DeviceCode>
                                        <DeviceCode>X</DeviceCode>

I want to test if certain code combinations exist such as "If <DeviceCode> = 
'A' and <DeviceCode> != 'B'"
Can someone please help with the best way to do this?
thanks in advance,
gary