So i have this solution for proving that the chromatic index of graph G, that is drawn on the image, equals 5.
I understand everything until that point, where i put "$?$" in circle. I don't know what that means, also the solution was orginally in a different language, so perhaps "$color$ $class$" isn't the right name for it, i used direct translation from slovene into english. Also that lemma means that $2m$ (where m is the number of edges) is equal to a product of number of vertices and number of edges in a single vertice, because the number of edges to every vertice is the same(4). If anyone can help me understand how we got to that inclusion, where i have a question mark, it would be great. Thank you in advance.