How can I use Nant's xmlpoke target to remove a node

Given the following xml:

<rootnode>
   <childnode arg="a">Content A</childnode>
   <childnode arg="b">Content A</childnode>
</rootnode>

Using XMLPoke with the following XPath:

rootnode/childnode[arg='b']

The result (if the replace string is empty) is:

<rootnode>
   <childnode arg="a">Content A</childnode>
   <childnode arg="b"></childnode>
</rootnode>

The contents of the childnode have been removed when we actually want the childnode itself removed. The desired result is:

<rootnode>
   <childnode arg="a">Content A</childnode>
</rootnode>

The childnode must be selected based on the childnode argument.