code: public CodingAnnotationStudy extractRaters(final int... raters) {
  		CodingAnnotationStudy result = new CodingAnnotationStudy(raters.length);
  		for (ICodingAnnotationItem item : getItems()) {
  			CodingAnnotationItem newItem = new CodingAnnotationItem(raters.length);
  			for (int r = 0; r < raters.length; r++) {
  				IAnnotationUnit unit = item.getUnit(raters[r]);
  				newItem.addUnit(result.createUnit(result.items.size(),
  						r, unit.getCategory()));
  			}
  			result.items.add(newItem);
  		}
  		return result;
  	}
docs: Returns a clone of the current annotation study which contains
  only the annotation units of the raters with the given indexes.
  All other units will be removed. This method is useful for
  converting an annotation study with multiple raters into a
  (pairwise) annotation study with two raters.