The cumulative distribution function = cdf of X a random variable with
distribution Px (support(Px)=(a,b) subset of R) = Fx:R->[0,1],
Fx(a)==Px(x<=a) forall a in R
Properties:
* Fx non-decreasing
* Fx(a)=0 forall x < a and Fx(b)=1 forall x>b
* Fx right continuous