    # a country type 

          declare Country

               name : String

            end

      

          # a city type holding a reference to its country

          declare City

               name : String

               country : Country

            end

      
    # meaning I want to collect $country objects and not City
      objects

    
    $countries : HashSet() from collect (City($name matches
      "X.*", $country : country), $country)

    
    # custom "buildset" accumulate function

    
    $countries : HashSet() from accumulate (City($name
      matches "X.*", $country : country), buildset($country))