Fix for describe() when axioms don't have a sourceCls_.
Fix for describe() when axioms don't have a sourceCls_.

jacksonic/vjlofvhjfgm,foam-framework/foam2,foam-framework/foam2,foam-framework/foam2,foam-framework/foam2,TanayParikh/foam2,TanayParikh/foam2,TanayParikh/foam2,TanayParikh/foam2,jacksonic/vjlofvhjfgm,foam-framework/foam2,jacksonic/vjlofvhjfgm