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<div class="fragment"><pre class="comment"> edge_color(g,algo) --- find a proper edge coloring of the graph g
 The returned matrix has three columns. Each row of this matrix of the
 form [i j c] where ij is an edge of the graph and c is a color number
 (from 1 to the number of colors used). 
 
 algo denotes an algorithm. The choices are:

 greedy:    use a greedy coloring, based on the line graph (default)
 
 optimal:   create an optimal coloring, based on the 