code: static public Result insertElements(Tree tree, List<TreeElement> elements, NowReference now) throws Exception {
  		
  		ResultImpl result = new ResultImpl(tree);
  		
  		TreeNodeRegular regularRootNode = tree.getRegularRootNode();
  		TreeNodeOngoing ongoingRootNode = tree.getOngoingRootNode();
  		for(TreeElement element : elements){
  			TimeInterval elementInterval = element.getInterval();
  			
  			if( elementInterval.isOngoing() ){
  				ongoingRootNode.insertElement(element, result, tree.getOperations(), now);
  			} else {
  				regularRootNode.insertElement(element, result, tree.getOperations(), now);
  			}
  		}
  		
  		return result;
  	}
docs: Modifies a cluster tree as a result of adding a new elements in the
  tree.
  @param tree Tree where the element is inserted.
  @param elements Elements to be inserted in the tree
  @return Results of inserting the elements
  @throws Exception