out_edges(0): (0,1) (0,2)
in_edges(0): (1,0) (2,0)
in a directed graph is (u,v) == (v,u) ? 0
weight[(u,v)] = 1.2
weight[(v,u)] = 2.4
in an undirected graph is (u,v) == (v,u) ? 1
weight[(u,v)] = 3.1
weight[(v,u)] = 3.1
the edges incident to v: (0,1)
