comparing two xml files using xslt

Lets say I have two XML files :

file "a" :

<?xml version="1.0"?>
 <catalog>
  <cd>d</cd>
  <cd>e</cd>
  <cd>f</cd>
  <cd>c</cd>
</catalog>

file "b":

<?xml version="1.0"?>
<catalog>
  <cd>a</cd>
  <cd>b</cd>
  <cd>c</cd>
</catalog>

I want to compare File b against File a and get those records which are only present in file b i.e. expected output is :

<?xml version="1.0"?>
 <catalog>
  <cd>a</cd>
  <cd>b</cd>
</catalog>