hull2d
The boolean operator <hull2d> specifies the hull of a set of 2d shapes contained. The result is a new 2d shape.
Example with the 2d hull of a circle and an offset smaller circle extruded to create a 3d solid.
<?xml version="1.0" encoding="utf-8"?>
<xcsg version="1.0">
<linear_extrude dz="1">
<hull2d>
<circle r="10"/>
<circle r="5">
<tmatrix>
<trow c0="1" c1="0" c2="0" c3="15"/>
<trow c0="0" c1="1" c2="0" c3="20"/>
<trow c0="0" c1="0" c2="1" c3="0"/>
<trow c0="0" c1="0" c2="0" c3="1"/>
</tmatrix>
</circle>
</hull2d>
</linear_extrude>
</xcsg>