/**
* Adds a new edge to a line, merging with other edges on this line if necessary
*
* @param edges : An EdgeLine map
* @param a line for this map, may already exist or not
* @param i first grid index for the nex edge on this line
* @param j second grid index for the nex edge on this line
*/
protected void addEdge(HashMap<EdgeLine, EdgeLine> edges, EdgeLine line, int i, int j) {
EdgeLine el = edges.get(line);
if (el == null) edges.put(line, line);
else line = el;
line.add(i, j);
}
