private List<String> getCoWords(String word) {
		Set<String> words = new HashSet<String>();
		List<Long> commentIds = words2Comments.get(word);
		if (commentIds == null || commentIds.size() == 0) {
			return new ArrayList<String>();
		}
		for (Long id: commentIds) {
			words.addAll(comments2Words.get(id));
		}
		return new ArrayList<String>(words);
	}