n a mouse-drop splice the new node in between the dropped-upon link's fromNode and toNode
      mouseDrop: (e, link) => {
        const oldto = link.toNode;
        const newnode = e.diagram.selection.first();
        if (!mayConnect(newnode, oldto)) return;
        if (!mayConnect(link.fromNode, newnode)) return;
        link.toNode = newnode;
        e.diagram.model.addLinkData({ from: newnode.key, to: oldto.key });
      }
    })
      .add(
        // two path Shapes: the transparent one becomes visible