need help with XSL statement (location path parenthesi

1)  The number of reports which are coded with either an "a" or "b"
where the last code (in document order) which is either an "a" or "b" is
an "a".

2)  The number of reports which are coded with either an "a" or "b"
where the last code (in document order) which is either an "a" or "b" is
a "b".

<?xml version="1.0" encoding="UTF-8"?>
<reportlist>
   <report>
       <code type="a"/>
       <code type="b"/>
       <code type="c"/>
   </report>
   <report>
       <code type="a"/>
       <code type="b"/>
       <code type="a"/>
   </report>
   <report>
       <code type="z"/>
       <code type="a"/>
       <code type="d"/>
   </report>
   <report>
       <code type="a"/>
       <code type="e"/>
       <code type="f"/>
   </report>
   <report>
       <code type="e"/>
   </report>
</reportlist>