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