A dataset including data elements and {HEAD ~ order keys} for indicating {TAIL ~ an ordering} of the data elements is stored 
for indicating
A dataset including data elements and { HEAD ~ order keys } for indicating { TAIL ~ an ordering } of the data elements is stored
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH