Author:	  Martin Maechler, ETH Zurich

Title:	  Class Hierarchies and Method Organization for Matrices

Abstract: In co-developing the R package "Matrix", we have used
	  S4 classes and methods throughout.  Using class
	  hierarchies based on three (sometimes even four)
	  different property classes was found to be
	  powerful. Since many methods apply to \emph{pairs} of
	  matrices, we have defined many methods for high level
	  (virtual) classes rather than for many or all (pairs of) 47
	  ``actual'' (non-virtual) classes.

	  Using the example of "Matrix", we show tools and draw
	  conclusions for more general class and method
	  management using S4, including inheritance
	  visualization and organization of testing code.