Constructions of optical queues by optical Switches and fiber Delay Lines (SDL) have received a lot of attention lately. In this short paper, we provide a simple proof for the construction of a priority queue with a switch and fiber delay lines in Sarwate and Anantharam [10]. Our proof not only gives the insights needed to understand why such a construction works, but also leads to a much general result that recovers the result in [10] as a special case. Moreover, our result can be easily extended to the constructions of priority queues with multiple inputs. Such constructions appear to be new in the literature.