code: protected void addUnit(final IAnnotationUnit unit) {
  		int raterIdx = unit.getRaterIdx();
  		if (raterIdx >= units.size()) {
  			for (int i = units.size(); i < raterIdx; i++) {
                  units.add(null);
              }
  			units.add(unit);
  		}
          else {
              units.set(raterIdx, unit);
          }
  
  		if (unit.getCategory() != null) {
              nonNullCount++;
          }
  	}
docs: Adds the given unit to the coding study. Normally, the method
  {@link CodingAnnotationStudy#addItem(Object...)} should be
  used to define the annotation units of an item.