In tut10.rt you can see, from left to right the subtraction, the
union and the intersection between two spheres.
You must give the two objects, like attributes :first and second. This
attribute values can be also other csg node.
The material is not needed, csg node use the material of the
associate objects.
Remember : objects in csg node must be always closed.
Look at file tut10.rt for an example of all tree case.