code: @SuppressWarnings("unchecked")
      public static <T> Stream<T> concat(Stream<? extends T> a,
                                         Stream<? extends T> b) {
          return new Stream(Iterators.concat(a.iterator, b.iterator));
      }
docs: Creates a concatenated stream whose elements are all the elements of the first stream followed by all the elements of the second stream.
  
  @param a   first stream
  @param b   second stream
  @param <T> type of elements
  @return a stream concatenating first and second