Smoothed versions of the intersection and union Boolean functions for implicits.
@article{fayolle2006constructive,
title={Constructive heterogeneous object modeling using signed approximate real distance functions},
author={Fayolle, Pierre-Alain and Pasko, Alexander and Schmitt, Benjamin and Mirenkov, Nikolay},
journal={Journal of Computing and Information Science in Engineering},
volume={6},
number={3},
year={2005}
}