said control means further operating to compare the stored gap value with {TAIL ~ stored values} representing {HEAD ~ the desired nominal gap value}; and to thereupon 
said control means further operating to compare the stored gap value with { TAIL ~ stored values } representing { HEAD ~ the desired nominal gap value } ; and to thereupon
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH