code: public CodingAnnotationStudy stripCategories(final Object keepCategory,
  			final Object nullCategory) {
  		CodingAnnotationStudy result = new CodingAnnotationStudy(getRaterCount());
  		for (ICodingAnnotationItem item : getItems()) {
  			CodingAnnotationItem newItem = new CodingAnnotationItem(raters.size());
  			for (IAnnotationUnit unit : item.getUnits()) {
  				Object newCategory;
  				if (!keepCategory.equals(unit.getCategory())) {
                      newCategory = nullCategory;
                  }
                  else {
                      newCategory = keepCategory;
                  }
  				newItem.addUnit(result.createUnit(result.items.size(),
  						unit.getRaterIdx(), newCategory));
  			}
  			result.items.add(newItem);
  		}
  		return result;
  	}
docs: Returns a clone of the current annotation study in which all categories
  are replaced by the given nullCategory except the categories matching
  the specified keepCategory.