code: @NotNull
      public IntStream flatMap(@NotNull final IntFunction<? extends IntStream> mapper) {
          return new IntStream(params, new IntFlatMap(iterator, mapper));
      }
docs: Returns a stream consisting of the results of replacing each element of
  this stream with the contents of a mapped stream produced by applying
  the provided mapping function to each element.
  
  <p>This is an intermediate operation.
  
  <p>Example:
  <pre>
  mapper: (a) -&gt; [a, a + 5]
  stream: [1, 2, 3, 4]
  result: [1, 6, 2, 7, 3, 8, 4, 9]
  </pre>
  
  @param mapper a non-interfering stateless function to apply to each
  element which produces an {@code IntStream} of new values
  @return the new stream
  @see Stream#flatMap(Function)