By means of the pair relationship a set of vertex pairs, e.g. (b, c) € , is defined. These vertex pairs can be ordered or nnordered. Unordered vertex pairs are called edges; in the following they are denoted as (b,e) € Gi, and are represented by a line coimecting the corresponding vertices. The geometric form of such a line is meaningless. Ordered vertex pairs, however, are called arcs and will be designated by square brackets and denoted by a directed line, where the arrow points from the first to the second vertex of the pair, e.g. (u,v) G G2’ Here, the vertex u is the starting point (source) and vertex v the final point (sink) of the arc (u, v). Moreover, the set £ may include elements which relate a vertex to itself, e.g. (s ,s ) € Gs; such elements are called loops.