Further, the topology service may create, from a specified topology, {TAIL ~ a directed acyclic graph} and {HEAD ~ corresponding closure table} 
Further , the topology service may create , from a specified topology , { TAIL ~ a directed acyclic graph } and { HEAD ~ corresponding closure table }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD