x1 (node/vertex/station) is backward of/along from x2 in oriented graph x3 (graph with orientation) using oriented edge path x4 (ordered sequence of ordered pairs; oriented edges). The path from x2 to x1 along the (now-unoriented version of the) edges used in x4 is counter/against/upstream of the orientation of x3 (and/or given by the oriented version of x4).
Orientation is given from/by x3, so only an edge need be submitted if x3 is fully specified; if x3 is not fully specified, x4 can take the burden of specifying the orientation and subgraph of particular interest (namely, the two vertices x1 and x2, intervening vertices along the path, and the orientation of the given edges connecting them). Note that on an unoriented edge or along a cycle, x1 and x2 might be able to exchange places and/or be equal one another. x4 is an ordered sequence of ordered pairs; the first entry of each pair is the origin node, the second pair is the destination node; the path should probably be connected (so that the destination node of one pair is the origin node of the next, except possibly if it is the last such pair). x1 is not necessarily last (id est: backward adjacent of/from) x2, but it can be. Useful for pages, webpages, family relationships, utterances, etc. See also: grafu, tcanaba, tcanaca