code: @SafeVarargs
      public static <T> Stream<T> parallelConcat(final Iterator<? extends T>... a) {
          return parallelConcat(a, DEFAULT_READING_THREAD_NUM, calculateQueueSize(a.length));
      }
docs: Put the stream in try-catch to stop the back-end reading thread if error happens
  <br />
  <code>
  try (Stream<Integer> stream = Stream.parallelConcat(a,b, ...)) {
  stream.forEach(N::println);
  }
  </code>
  
  @param a
  @return