As i said, I want to use $lookup as a Set so that I can later check against other values ​​using contains. For more clarity, this is the actual rule:

import java.util.Set;
global java.util.Map lookupValues;
dialect "java"
rule "ruleName"
when
$fact : MyFact($systematic : systematic)
$lookup : Set() from lookupValues["systematic"]
eval($lookup contains $lookup)

then
System.out.println("Error");

end

So, my question ist, whats wrong with the declaration of $lookup? Or is this something, Drools can't handle?