targetFunctions = new HashSet<Function>();
// add EqualFunction
targetFunctions.addAll((new EqualFunctionCluster()).getSupportedFunctions());
// add LogicalFunction
targetFunctions.addAll((new LogicalFunctionCluster()).getSupportedFunctions());
// add NOfFunction
targetFunctions.addAll((new NOfFunctionCluster()).getSupportedFunctions());
// add NotFunction
targetFunctions.addAll((new NotFunctionCluster()).getSupportedFunctions());
// add ComparisonFunction