The constraint solver used in Phun is based on SPOOK, and calculates impulses at contacts and hinges to satisfy the constraints. SPOOK is the work of Claude Lacoursière from his PhD thesis, Ghosts and machines: regularized variational methods for interactive simulations of multibodies with dry frictional contacts, Dept. of Computing Science, Umeå university, 2007. The resulting time stepper is closely related to the Leapfrog integration scheme. Claude Lacoursière works part time for Umeå university and Algoryx.