code: @SuppressWarnings("unchecked")
      public static <T> Collector<Optional<T>, List<T>, List<T>> flatList()
      {
          return (Collector) FLAT_LIST;
      }
docs: A {@link Collector} that collects {@linkplain Optional optional} values to a list.
  The collector only collects values that are {@linkplain Optional#isPresent() present}.
  
  @param <T> the type of values to collect.
  @return a collector that collects optional values to a list.