code: public CallTreeNode onStopwatchStart(Split split) {
  		final String name = split.getStopwatch().getName();
  		CallTreeNode currentNode;
  		if (callStack.isEmpty()) {
  			// Root tree node
  			rootNode = new CallTreeNode(name);
  			currentNode = rootNode;
  			onRootStopwatchStart(currentNode, split);
  		} else {
  			// Child node
  			currentNode = callStack.getLast().getOrAddChild(name);
  		}
  		callStack.addLast(currentNode);
  		return currentNode;
  	}
docs: When stopwatch is started, a new tree node is added to the parent
  tree node and pushed on the call stack.
  As a result, child tree node becomes the current tree node.
  
  @return Current (child) tree node