I am a little bit confused as I don't get what I am doing wrong. Here is my rule:

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rule "Exchange Pattern"
    when
        $p2: ProcessAdapter()
        $mainProcess: ProcessAdapter( isMainProcess(), this!=$p2)
        MessageFlowAdapter ( fromProcess == $mainProcess, toProcess==$p2)
        MessageFlowAdapter ( fromProcess == $p2, toProcess==$mainProcess)
        exists ItemAwareElementAdapter( ) from $mainProcess.getReceivedResources($p2)
        exists ItemAwareElementAdapter( ) from $mainProcess.getGivenResources($p2)       
    then
        System.out.println("** FOUND EXCHANGE PATTERN betwwen "+$mainProcess.getParticipant().getName()+" AND "+$p2.getParticipant().getName());
        System.out.println("Given: "+$mainProcess.getGivenResources($p2)+$mainProcess.getGivenResources($p2).size());
        System.out.println("Received: "+$mainProcess.getReceivedResources($p2)+$mainProcess.getReceivedResources($p2).size());
        System.out.println("Intersection: "+$mainProcess.intersectGivenAndReceivedResourcesWithProcess($p2)+$mainProcess.intersectGivenAndReceivedResourcesWithProcess($p2).size());
end
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