The food can only be eaten if a philosopher holds a fork in each hand. There is a fork between each pair of philosophers, so there are exactly as many forks as there are philosophers. The objective is to make the philosophers behave so that they all get a fair chance to eat. These numbers will be fairly close to each other, illustrating the fairness of the algorithm. Still, some Lisp vendors include series in their product because customers came to depend upon it during the time between the publication of CLtL2 and the ANSI Specification.
Series combine the behaviors of sequences, streams and loops.
Using series, you can write iterative code using a functional notation. Control is achieved by selecting or filtering elements as they pass through a series of filters and operators. The best place to find information and examples is in Appendix A of CLtL2. This book may not be reproduced without the written consent of its author. Online distribution is restricted to the author's site.