Re: Re: questions on simple profile - nested spans

I have a question, too.
http://dvcs.w3.org/hg/ttml/raw-file/tip/ttml10-sdp/Overview.html

The document presents this as an example for rollup captions:

<p region='rollup' begin='00:00:13.000' dur='00:00:11.000'>  <span
begin='00:00:01.000'>rollup</span>  <span
begin='00:00:02.000'>style</span>  <span
begin='00:00:03.000'>caption</span>  <span
begin='00:00:04.000'>support</span></p><p region='rollup'
begin='00:00:17.000' dur='00:00:11.000'>  <span
begin='00:00:01.000'>with</span>  <span
begin='00:00:02.000'>word</span>  <span begin='00:00:03.000'>at</span>
 <span begin='00:00:04.000'>a</span>  <span
begin='00:00:05.000'>time</span>  <span
begin='00:00:06.000'>temporal</span></p><p region='rollup'
begin='00:00:23.000' dur='00:00:11.000'>  <span
begin='00:00:01.000'>placement</span>  <span
begin='00:00:02.000'>this</span>  <span
begin='00:00:03.000'>could</span>  <span
begin='00:00:04.000'>go</span></p><p region='rollup'
begin='00:00:27.000' dur='00:00:11.000'>  <span
begin='00:00:01.000'>on</span>  <span begin='00:00:02.000'>all</span>
<span begin='00:00:03.000'>day</span></p>


Would you mind explaining how this works?
I am particularly confused about where the rollup happens. I assume
that the words within the <p> element are all on the same line, but
appear one at a time.

That leads me to the conclusion that each <p> will be rendered on the
same default position. Does that mean that the <p> elements push each
other out of the way?
How does that work technically: when you add a <p> on top of an
existing <p> in the same location, is it added below and the previous
<p> pushed upwards?

I wasn't able to find that specified in the document. Sorry for my ignorance.

Regards,
Silvia.