My previous example would have been nice if the UT hadn't trimmed off all my spaces. :(

Here is another try....

If I reply to your message, as I am doing, then the thread shows the following:

Your message
-tab-Michel's Message
-tab-My Message

Had I responded to Michel's Message, then the thread would have appeared as follows:

Your Message
-tab- Michel's Message
-tab--tab- My Message

In the follwoing example,
1
-tab- 2
-tab--tab-3
-tab--tab-6
-tab-4
-tab--tab-5

2 and 4 are responses to 1
3 and 6 are responses to 2
5 is a response to 4