graph file format

figure 2 in page 11 of the manual but it does not make any sense.
how is connectivity represented using the neighboring relationships?

also does it matter if I don't use consecutive node numbers? suppose I don't have node number 2 but I have node 3?

apparently I should put one entry per edge since edge (i,j) and (j,i) show the same thing. if all of the edges to node x are represented using (...,x) should i keep the line for node x?

RE: So my reading is that each

So my reading is that each edge should be respresented once and once all edges to a node are provided in the some other lines then a single entry for the node number is all i should put the file.

RE: Ignoring the header line,

Ignoring the header line, each other line stores the adjacency list of a vertex. The ith line (assuming the first line is numbered 0) stores the adjacency list of the ith vertex. The numbering of the vertices should be consecutive; i.e., you cannot skip vertex numbers. If a vertex does not have any other vertices adjacent to it, its corresponding line should be empty. Since the file stores the adjacency lists, for each edge (i,j), there should be an entry in the ith and the jth line of the file.