Therefore, as each vertex of the network graph is added, {HEAD ~ a series} of {TAIL ~ constraints} for that vertex of the graph is evaluated 
of
Therefore , as each vertex of the network graph is added , { HEAD ~ a series } of { TAIL ~ constraints } for that vertex of the graph is evaluated
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH