// all the tasks, then the thread will block waiting for all
            // those subtasks to finish.
            // This ensures the number of running workers is no more than
            // the defined parallism, while making sure the pool will not
            // be exhausted
            //
            return new ThreadPoolExecutor(

                    problems,
                    Severity.FATAL,
                    Version.V20,
                    "modelVersion",
                    null,
                    "of '" + string + "' is newer than the versions supported by this version of Maven: "
                            + validVersions + ". Building this project requires a newer version of Maven.",
                    tracker);