A single-server exponential queueing system of finite capacity with non-preemptive priority and negative customers

A single server queueing system with two customers flows, separated buffers of limited capacities, and non preemptive priority has been analyzed. Beside positive customers, a negative customers flow comes into the first buffer. A negative customer has the effect to remove a customer in the first buffer if it is not empty, otherwise he removes the customer coming from the first flow being served if there is one. Recurrent formulas and numerical results have been obtained for steady state probabilities