[xsl] Creating lists and sublists from a level attribute

Hi,

I currently have a series of <heading> tags in the xml file. These are not 

embedded within each other, but have a level attribute to indicate which 
ones are sub headings of others. For instance:

<heading level="1">A main Heading</heading>
<heading level="2">Sub heading of main heading</heading>
<heading level="2">another sub heading of main heading</heading>
<heading level="1">Another main Heading</heading> 

There can be up to 4 levels.

What I am trying to achieve is a "document outline" at the top of the page 

that takes each heading and puts it in a list, with sub headings being 
sublists of their parent headings. I am struggling to find a way to 
arrange the <ul> and <li> tags to achieve this, and any help would be much 

appreciated.

thanks, 

Andy