The features presented in this section are undoubtedly not for the casual PVM user, but for tool developers. Collaboration with other research groups and the addition of these features has had a positive effect on PVM. The protocols are now conceptually ``cleaner'' than they were before. We have shown that the system can be made more powerful and general without increasing the amount of code, by making it dynamically extensible. The result has been directly useful to all the projects involved, and we hope it will have even more widespread application as the interfaces become more stable and other researchers take advantage of them.