When {HEAD ~ the known code portion} is detected from {TAIL ~ the coded data}, at least a part thereof will be replaced with normal data, and the decoding will be performed on the coded data after the substitution 
is detected from
When { HEAD ~ the known code portion } is detected from { TAIL ~ the coded data } , at least a part thereof will be replaced with normal data , and the decoding will be performed on the coded data after the substitution
OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL 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