Link getLinkToNode(Node toNode) {
   if (!toNodes.containsKey(toNode)) {
     toNodes.put(toNode, getNewLink(myNode, toNode));
   }
   return toNodes.get(toNode);
 }
 Link getLinkFromNode(Node fromNode) {
   if (!fromNodes.containsKey(fromNode)) {
     fromNodes.put(fromNode, getNewLink(fromNode, myNode));
   }
   return fromNodes.get(fromNode);
 }