This class implements a random variable in the classic discrete probability theory,
which includes:
(1) a name for this random variable
(2) a set of possible states this random variable can take
Given a set of random variables "Y = {V1, ..., Vn}" from a Bayesian Belief Network,
retrieves the loose closure of "Y", which are defined as: "S = {Pi(V1), ..., Pi(Vn)}\Y".
Given a set of random variables "Y = {V1, ..., Vn}" from a Bayesian Belief Network,
retrieves the strict closure of "Y", which are defined as:
Initially, "S = {Pi(V1), ..., Pi(Vn)}\Y";
If some "Si" in "S" is a descendant of some "Yi" in "Y", then:
(1) "Y = Y + {Si}",
(2) "S = S - {Si} + Pi(Si)\Y".
Repeat this process until such a "Si" does not exist any more.