block length

Hello,

I would be grateful for a suggestion. Here is my problem:

Let us have a piece of XSL-FO code like this:

<flow>
	...
	<block id="b1"> ... </block>
	...
	<block id="b2" margin-left="???"> ... </block>
	
</flow>

How can I force the "b2" block to have a left margin equal to the length of
the
"b1" block?

Thank you in advance...