difference3d
The boolean operator <difference3d> specifies the difference between the first solid contained and the subsequent ones. The result is a new solid.
Example of 3d difference - a sphere subtracted from a centered cube.
<?xml version="1.0" encoding="utf-8"?>
<xcsg version="1.0">
<difference3d>
<cube size="50" center="true"/>
<sphere r="33"/>
</difference3d>
</xcsg>