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