Xml/Xslt - XML Design

Hi, 

I'm having some trouble with designing an XML document/vocabulary.

Situation:

I have sets of numerical data, that is essentially tabular data captured in xml elements:

<dataset id='1'>
<row/>
<row/>
<row/>
<dataset>
<dataset id='2'>
...
...
</dataset>
...

PROBLEM:
I need to extend this with xml that captures multiple hierarchical relationships between these datasets. Currently I've defined a <datahierarchy/> element that captures a single type of relationship between the datasets:

<datahierarchy>
<tier id='1'>
<dataset>1</dataset>
<dataset>2</dataset>

<tier id='2'>
<dataset>3</dataset>
</tier>
</tier>
...
</datahierarchy>

However, the datasets have several different hierarchal relationships. i.e. where <dataset id='3'> may be the root rather than a child, and have <dataset id='2'> and <dataset id='1'> as children. 

The easy solution would be to capture the different types of data hierarchies in multiple <datahierarchy></datahierarchy> elements. But I am reluctant to do this because it seems like a clumsy and spactially expensive solution. 

Any ideas on how I could capture the hierarchy and relationships of my datasets in xml? 

Could someone provide links to tutorials regarding xml design? I've googled it with very little sucess.

Thanks.

adding an optional <child> tag within each <dataset>:

<dataset id='1'> 
<row/> 
<row/> 
<row/> 
<child>
<childdataset id='2'>
<childdataset id = '3'>
</child>
<dataset> 
<dataset id='2'> 
... 
... 
</dataset> 
... 

I do have a question though.  Do the relationships just based on the dataset id?  Or it's based on some Key value(or attribute) on the row elements?