HashSet<Variable> joinIntersection = new HashSet<Variable>(firstOperand.getUnionVariables());
joinIntersection.retainAll(secondOperand.getUnionVariables());
join.setIntersectionVariables(joinIntersection);
firstOperand.addSucceedingOperator(join, 0);
secondOperand.addSucceedingOperator(join, 1);
jointree.add(new Tuple<BasicOperator, T>(join, mergeInitialInformations(firstSubGraph.getSecond(), secondSubGraph.getSecond())));
}
BasicOperator op = jointree.get(0).getFirst();