Consider a multi$-$user operating system, such as UNIX. The memory allocator manages a finite number of consecutive memory blocks, which users may want to access. The memory allocator maintains a directory of which blocks are owned by which users. The following constraints must be observed

a$)$ No block is owned by more than one user.

b$)$ A user may own more than one blocks.

c$)$ Some blocks might not be owned at all.

d$)$ Some users might not own any blocks