Customers arrive at a service facility according to a Poisson process of rate
\lambda customers per hour. Let N(t) be the number of customers that have
arrived up to time t, and let W_(1),W_(2),dots be the successive arrival times of the
customers. Determine the expected value of the product of the waiting
times up to time t. Assume that prod_(j)=1^(N(t))W_(j)=1 when N(t)=0.