In calculating {HEAD ~ a route} through a network of nodes and links, a standard constrained shortest path first method builds a network graph, accumulating a set of {TAIL ~ path data} 
In calculating { HEAD ~ a route } through a network of nodes and links , a standard constrained shortest path first method builds a network graph , accumulating a set of { TAIL ~ path data }
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL