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5.18.2. declare-error

      <declare-error expr="call(* org.acme.businesslayer.*->*(..)) \
            AND within(org.acme.datalayer.*)">
      Data layer classes should not call up to the business layer
   </declare-error> 

The above declaration says that if any classes in the datalayer call classes in the business layer of your application, an error should be thrown. Instumentation/execution of your application will stop.