Exponential probability distribution function.
Exponential(g,beta)
Generates a random variate following the Exponential probability distribution of mean beta.
The pseudo-code algorithm used for this distribution is:
1. Generate u as U(0,1)
2. Return x = -beta * ln(1-u)
(u,g) := Exponential(g,0.4);
function Exponential extends Var; end Exponential;