Open Systems
By an "open system" we mean one that is extensible and can interoperate with other systems. Such systems go beyond the notion of a tool kit, which is extensible but not necessarily "open." They provide well defined interfaces to other components. Nuprl 3 was a closed tactic based tool kit. Nuprl 5 is built in such a way that there are well defined interfaces for connecting editors, logic engines, evaluators, compilers, symbolic algebra systems and so forth. Notions of modularity are key to achieving openness, and we use modules and objects to this end.