Modify the S4 class, and add test case. Refs: (1) http://journal.r-project.org/archive/2011-1/RJournal_2011-1_Wickham.pdf (2)http://cran.r-project.org/doc/contrib/Leisch-CreatingPackages.pdf (3)http://www.r-project.org/conferences/useR-2004/Keynotes/Leisch.pdf (4) http://www.bioconductor.org/help/course-materials/2011/AdvancedRFeb2011Seattle/ImplementingS4Objects-lab.pdf (5) http://cran.r-project.org/web/packages/Brobdingnag/vignettes/brob.pdf