code: public static <T> Iterator<T> concat(Iterator<? extends Iterator<? extends T>> iterators) {
          Preconditions.checkArgumentNotNull(iterators, "iterators");
          return new CombinedIterator<T>(iterators);
      }
docs: Combine the iterators into a single one.
  
  @param iterators An iterator of iterators
  @return a single combined iterator