gives the number of edges in the graph g.
is obsolete because M[g] works for directed as well as undirected graphs.
Wolfram Research (2012), M, Wolfram Language function, https://reference.wolfram.com/language/Combinatorica/ref/M.html.
Wolfram Language. 2012. "M." Wolfram Language & System Documentation Center. Wolfram Research. https://reference.wolfram.com/language/Combinatorica/ref/M.html.
Wolfram Language. (2012). M. Wolfram Language & System Documentation Center. Retrieved from https://reference.wolfram.com/language/Combinatorica/ref/M.html