 create and add VirtualizedTreeEdge
            const e = this.createEdge();
            e.data = child; // associate this Edge with data, not a Link
            e.fromVertex = f;
            e.toVertex = t;
            this.addEdge(e);
          }
        }
      } else {
        throw new Error('can only handle TreeModel data');
      }
    }
  }
  // end VirtualizedTreeNetwork class

  class VirtualizedTreeEdge extends go.TreeEdge {
    constructor(network) {
      super(network);
    }

    static _dumm