{HEAD ~ The files} are grouped into {TAIL ~ at least one set} of files, where the files in the set are accessed together more than a predetermined number of times in the trace 
{ HEAD ~ The files } are grouped into { TAIL ~ at least one set } of files , where the files in the set are accessed together more than a predetermined number of times in the trace
HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL 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