Estimation, testing and regression modeling of subdistribution
functions in competing risks, as described in Gray (1988), A class
of K-sample tests for comparing the cumulative incidence of a
competing risk, Ann. Stat. 16:1141-1154, and Fine JP and Gray RJ
(1999), A proportional hazards model for the subdistribution of a
competing risk, JASA, 94:496-509.