Exercise $2\mathrm{.}2.$ Why must we define a doorway section, rather than defining first$-$ come$-$first$-$served in a mutual exclusion algorithm based on the order in which the first instruction in the lock$()$ method was executed? Argue your answer in a case$-$by$-$ case manner based on the nature of the first instruction executed by the lock$()$: a read or a write, to separate locations or the$$same$$location$.$