convert following-sibling to be childs

hi
in a   big xml database i need  to turn some following sibling elements
into childs of that element:

if we have the following database :

<top>
<set1/><x/><y/><y/><x/><y/><set2/><x/><y/><x/><y/><x/><y/><y/><x/><y/>
<set3/><x/><x/><y/><x/><y/>x/><y/><x/><y/><x/><y/>

</top>

i want all the following sibling  <x/>and <y/> of the element <set1> until
we reach the <set2/> to be childs of <set1/>
the same with <x/> and <y/> following set2
and the same with <x/> and <y/> following <set3/>