docs: public <U> FutureStream<U> iterate(final U seed, final UnaryOperator<U> f) {
  
          final Subscription sub = new Subscription();
          final Supplier<U> supplier = new Supplier<U>() {
              @SuppressWarnings("unchecked")
              U t = (U) NONE;
  
              @Override
              public U get() {
                  return t = t == NONE ? seed : f.apply(t);
              }
          };
          return construct(StreamSupport.<U> stream(new InfiniteClosingSpliteratorFromSupplier<U>(
                                                                                                  Long.MAX_VALUE, supplier, sub),
                                                    false));
  
      }