public Skellam(double mu1, double mu2) throws ParameterException {
		if (mu1 <= 0 || mu2 <= 0) {
			throw new ParameterException("Skellam parameters mu1, mu2 > 0.");
		} else {
			this.mu1 = mu1;
			this.mu2 = mu2;
			pois1 = new Poisson(mu1);
			pois2 = new Poisson(mu2);
		}
	}