Suggest an efficient scheme for maintaining a per-user protection scheme.

That is, the system maintains one directory per user, and that directory lists all

the objects to which the user is allowed access. Your design should address the

needs of a system with 1000 users, of whom no more than 20 are active at any

time. Each user has an average of 200 permitted objects; there are 50,000 total

objects in the system.