$($d$)$ We modelled the amended protocol by a process System $(t),$ specified its correctness by a process SPEC,

and checked its correctness by a suitable assertion:

send, receive $|$ USERS

SPEC$=$ know $?n$ : noncesI $$SPEC

assert SPEC $[$T$=$ System$(20)$ send, receive $|$

In the following, 'explain' means giving an informal explanation showing an understanding of the mod$-$

elling.

$($i$)$ Explain what the process System$(t)$ means $($describe its components and explain the way they are

connected$).$

$($ii$)$ Explain what the assertion means. Explain all parts of it$,$ in particular, explain the meaning of

I II send, receive $1$