code: public final static <T, R> Stream<R> flatMapOptional(final Stream<T> stream, final Function<? super T, Optional<? extends R>> fn) {
          return stream.flatMap(in -> Streams.optionalToStream(fn.apply(in)));
  
      }
docs: cross type flatMap, removes null entries
  <pre>
  {@code
  assertThat(Streams.flatMapOptional(Stream.of(1,2,3,null),
  Optional::ofNullable)
  .collect(CyclopsCollectors.toList()),
  equalTo(Arrays.asList(1,2,3)));
  
  }
  </pre>