code: public static <T> boolean verifyAllSuccessful(List<Either<T, ExecutionException>> results) {
      return Iterables.all(results, new Predicate<Either<T, ExecutionException>>() {
        @Override
        public boolean apply(@Nullable Either<T, ExecutionException> input) {
          return input instanceof Either.Left;
        }
      });
    }
docs: Utility method that checks whether all tasks succeeded from the output of {@link #executeAndGetResults()}.
  @return true if all tasks succeeded.