help in xslt2 grouping

hi all,
I need to group in XSLT2 'elem' that have a child 'title'.
The XML is:

<root>
 <elem><title>row1</title></elem>
 <elem>row2</elem>
 <elem><title>row3</title></elem>
 <elem><title>row4</title></elem>
 <elem>row5</elem>
 <elem>row6</elem>
</root>

<root>
<elem>
  <title>row1</title>
  <content>row2</content>
</elem>
<elem>
  <title>row3</title>
  <content>row4</content>
  <content>row5</content>
</elem>
</root>

Regards,
alex.