code: function _safeGetEdge(g, u, v) {
      var key = _edgeKey(u, v);
      if (!(key in g._edges)) {
        throw new Error("No such edge: (" + u + ", " + v + ")");
      }
      return g._edges[key];
    }
docs: Returns the edge object for (u, v) in E or throws an error if the edge does not exist.