https://sites.temple.edu/pdames/files/2016/07/DamesTokekarKumarISRR2015.pdf
Let Xt = fx1;t;x2;t;:::;xnt;tg denote a realization of a RFS of target states at time t. A probability distribution of a RFS is characterized by a discrete distribution over the cardinality of the set and a family of densities for the elements of the set conditioned on the size, i.e.,