public List<Tuple<BigInteger, BigInteger>> getNegSentences(int lengthPos) {

		List<Tuple<BigInteger, BigInteger>> result = new ArrayList<Tuple<BigInteger, BigInteger>>();
		ListIterator<Tuple<Annotation,Boolean>> senIter = sentences.listIterator();		
		Tuple<Annotation,Boolean> currSen = null;
		
		for (int i = 0; i < lengthPos && senIter.hasNext(); i++) {

			currSen = senIter.next();

				if (!currSen.y) {
					Tuple<BigInteger, BigInteger> sentenceTuple = new Tuple<BigInteger, BigInteger>(
							currSen.x.getStartNode(), currSen.x.getEndNode());
					result.add(sentenceTuple);
				}
		}
		return result;
	}