public static String subtask3(CompositeText document, int sentenceIndex) {
		List<TextComponent> sentences = listOfAllSentences(document);
		TextComponent theSentence = sentences.get(sentenceIndex);
		
		Set<String> words = new TreeSet<String>();
		for(TextComponent token : theSentence){
			if (token instanceof Token && token.getType().equals(TokenType.WORD)) {
				words.add(token.getValue());
			}
		}
		Set<String>  documentWords = new TreeSet<String>();
		for(int i=0;i<sentences.size();i++){
			if(i==sentenceIndex){
				continue;
			}
			for(TextComponent token:sentences.get(i)){
				if (token instanceof Token && token.getType().equals(TokenType.WORD)) {
					documentWords.add(token.getValue());
				}
			}
		}
		words.removeAll(documentWords);
		return words.toString();
	}