Once {HEAD ~ the portion} of the carpet strip that comes into contact with {TAIL ~ the elevator ropes} becomes soiled, the carpet strip can be advanced so that a clean portion of the carpet strip is positioned to be in contact with the elevator ropes and the soiled portion of the carpet strip is discarded 
Once { HEAD ~ the portion } of the carpet strip that comes into contact with { TAIL ~ the elevator ropes } becomes soiled , the carpet strip can be advanced so that a clean portion of the carpet strip is positioned to be in contact with the elevator ropes and the soiled portion of the carpet strip is discarded
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH