code: public static ConcurrentLimitRule of(int concurrentLimit, TimeUnit timeOutUnit, long timeOut) {
          requireNonNull(timeOutUnit, "time out unit can not be null");
          return new ConcurrentLimitRule(concurrentLimit, timeOutUnit.toMillis(timeOut));
      }
docs: Initialise a concurrent rate limit.
  
  @param concurrentLimit The concurrent limit.
  @param timeOutUnit     The time unit.
  @param timeOut         A timeOut for the checkout baton.
  @return A concurrent limit rule.