code: @NotNull
      public LongStream flatMapToLong(@NotNull final Function<? super T, ? extends LongStream> mapper) {
          return new LongStream(params, new ObjFlatMapToLong<T>(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.
  
  @param mapper  the mapper function used to apply to each element
  @return the new {@code LongStream}
  @see #flatMap(com.annimon.stream.function.Function)