buildingenter()
s1.wait
NR.wait
	D.wait
		d1.lock
		s1.release
	D.release
	D.wait
		d2.lock
		d1.unlock
	D.release
	D.wait
		d3.lock
		d2.unlock
	D.release
	D.wait
		d3.unlock
	puts 'entered building'
NR.release


buildingleave()
s2.wait
...
puts 'left building'
	

rotate()
while
	NR.wait
		if(d1/d2/d3/d4)	rotate90
	NR.release	



main()
sem s1=30 (enter queue length of worker threads)
sem s2 (exit queue)