code: public <B> FutureT<W,B> flatMapT(final Function<? super T, FutureT<W,B>> f) {
          return of(run.map(future -> future.flatMap(a -> f.apply(a).run.stream()
                                                                        .toList()
                                                                        .get(0))));
      }
docs: Flat Map the wrapped Future
  <pre>
  {@code
  FutureT.of(AnyM.fromStream(Arrays.asFuture(10))
  .flatMap(t->Future.completedFuture(20));
  
  
  //FutureT<AnyMSeq<Stream<Future[20]>>>
  }
  </pre>
  @param f FlatMap function
  @return FutureT that applies the flatMap function to the wrapped Future