code: @NotNull
      public static DoubleStream concat(
              @NotNull final DoubleStream a,
              @NotNull final DoubleStream b) {
          Objects.requireNonNull(a);
          Objects.requireNonNull(b);
          DoubleStream result = new DoubleStream(new DoubleConcat(a.iterator, b.iterator));
          return result.onClose(Compose.closeables(a, b));
      }
docs: Concatenates two streams.
  
  <p>Example:
  <pre>
  stream a: [1, 2, 3, 4]
  stream b: [5, 6]
  result:   [1, 2, 3, 4, 5, 6]
  </pre>
  
  @param a  the first stream
  @param b  the second stream
  @return the new concatenated stream
  @throws NullPointerException if {@code a} or {@code b} is null