def add_edge (u, v)
      add_vertex(u)                         # ensure key
      add_vertex(v)                         # ensure key
      basic_add_edge(u, v)
    end
def add_edge (u, v)
      add_vertex(u)                         # ensure key
      add_vertex(v)                         # ensure key
      basic_add_edge(u, v)
    end
See MutableGraph#add_edge.