code: public static <A, B> BaseListCommand<A, B> flatAnd(Command<A, ? extends Iterable<B>> first, Command<A, B> second) {
          return new FlatAndCommand<>(first, second);
      }
docs: Creates a new task that will append the result of the second task to the result-list
  of the first.
  TODO Fabian: test behavior and explain here.