Employing a binary semaphore, s: semaphore :$=(0,),$ the tasks X and Y can be implemented sequentially as follows:Figure $1\mathrm{.}18.$ A dependency graph for tasks A$,$ B$,$ C$,$ and D$.$Due to the use of semaphore s operations of task Y will be executed after operations of task X$.$ Given the tasks A$,$ B$,$ C$,$ and D synchronize their execution by using semaphores according to the dependency graph depicted in Figure $1\mathrm{.}18.$ As can be seen, the first task to execute is A$,$ then tasks B and C should be executed in parallel, and finally task D$.$