In retrospect, it appears that a method that defines fluid regions rather than interfaces offers the advantage of logical simplicity for situations involving interacting multiple free boundaries. While the marker particle method provides this simplicity, it suffers from a significant increase in required computer storage. It also requires additional computational time to move all the points to new locations. It is natural, therefore, to seek an alternative that shares the region defining property without an excessive use of computer resources. Such a method is described in the next section