When {HEAD ~ the value} of the counter reaches a prescribed value, which corresponds to {TAIL ~ the point} at which substantially all the unburned particulates on the exhaust duct have been combusted, both the first and second counters are reset 
When { HEAD ~ the value } of the counter reaches a prescribed value , which corresponds to { TAIL ~ the point } at which substantially all the unburned particulates on the exhaust duct have been combusted , both the first and second counters are reset
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH 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