When portability of the software is important, UI interface must be carefully planed. Common approach to UI design is to separate CSG kernel from 3D and toolbox window. Inter-process communication can be established for data transfer between modules. Size of data transfer is important for response time. Thats why some CSG kernels are with close relationship to display module. Toolbox and other UI related communication is not critical and can be implemented with standard link such as named pipes (FIFO), IPC, RPC mechanisms.
Extensibility of built functions is important for adding derived primitives to kernel. Only very basic primitives and thus robust kernel can be built if derived object can be used in the same manner as basic primitives. Builtin language for user macros can be interpreter or compiler with shared libraries. As CSG tree is used some list processing language such as lisp or scheme is practical and easy to use for user macros.