docs: @GuardedBy("elementsLock")
  	@Override
  	public boolean markChecked(CandidateElement element) {
  		String generalString = element.getGeneralString();
  		String uniqueString = element.getUniqueString();
  		synchronized (elementsLock) {
  			if (elements.contains(uniqueString)) {
  				return false;
  			} else {
  				elements.add(generalString);
  				elements.add(uniqueString);
  				return true;
  			}
  		}
  	}