code: @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;
  			}
  		}
  	}
docs: Mark a given element as checked to prevent duplicate work. A elements is only added when it
  is not already in the set of checked elements.
  
  @param element the element that is checked
  @return true if !contains(element.uniqueString)