The passage can be {TAIL ~ a closed slot}, {HEAD ~ open notch}, or other structure on which the belt travels 
The passage can be { TAIL ~ a closed slot } , { HEAD ~ open notch } , or other structure on which the belt travels
OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH