code: @Pure
      public static double random(double lambda, double xmin) throws MathException {
          return StochasticGenerator.generateRandomValue(new ExponentialStochasticLaw(lambda, xmin));
      }
docs: Replies a random value that respect
  the current stochastic law.
  
  @param lambda is the parameter of the distribution.
  @param xmin is the x coordinate where {@code f(x)=lambda}
  @return a value depending of the stochastic law parameters
  @throws MathException when error in the math definition.