negating a class

[Vilius Puidokas]

got cfengine 1.5.4
having trouble to negate a class in section groups:

groups:
        product = ( "/usr/bin/test -e /etc/somefile" )

now if wanna have groups which is !product i try doing:
        non-product = ( !product )
        non-product = ( -product )

doesn't look to work:
-group needs +group, 
!group complains about 'Wildcards cannot be used in this context')

any other way? (say, without doing inverted shell-check i.e. inside
cfengine)