considering controlled burns of oil in the reservoir to raise pressure and
temperature. These are much more complex problems, though, and left for future
projects.
+
+Finally, from a mathematical perspective, we have derived the
+criterion for re-computing the velocity/pressure solution at a given
+time step under the assumption that we want to compare the solution we
+would get at the current time step with that computed the last time we
+actually solved this system. However, in the program, whenever we did
+not re-compute the solution, we didn't just use the previously
+computed solution but instead extrapolated from the previous two times
+we solved the system. Consequently, the criterion was pessimistically
+stated: what we should really compare is the solution we would get at
+the current time step with the extrapolated one. Re-stating the
+theorem in this regard is left as an exercise.