<A>
  <B><C/><C/>...<C/></B>
  <B><C/><C/>...<C/></B>
   .
   .
</A>

Thanks to Joerg Heinicke, I managed to pass the position of
the B element to the C elements. Further, I now need a globally
unique number for C. For that, I've been using (within C):

But this seems very inefficient, and it looks like performance
degrades over time (may be an implementation issue?). Any hints
on a less costly (constant time perhaps) way of getting
enumerated the C elements?