repeating {HEAD ~ the sequence} of steps through with respect to {TAIL ~ subsequent substrate elements} in the series 
repeating { HEAD ~ the sequence } of steps through with respect to { TAIL ~ subsequent substrate elements } in the series
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH