combining classes

[D'Ausilio, John]

I define some classes in a common bundle representing groups of hosts:

class_a or => {classify("a1"),classify("a2"),classify("a3")};
class_b or => {classify("b1"),classify("b2"),classify("b3")};

I can't seem to find a way to combine the two classes other than the
trivial solution

shouldn't something like this work?

class_ab or => {class_a, class_b};

or even ?

class_ab expression => (class_a|class_b);

thanks
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