code: Map<String, UserData> getUserDataMap(NodeImpl node) {
          if (nodeToUserData == null) {
              nodeToUserData = new WeakHashMap<NodeImpl, Map<String, UserData>>();
          }
          Map<String, UserData> userDataMap = nodeToUserData.get(node);
          if (userDataMap == null) {
              userDataMap = new HashMap<String, UserData>();
              nodeToUserData.put(node, userDataMap);
          }
          return userDataMap;
      }
docs: Returns a map with the user data objects attached to the specified node.
  This map is readable and writable.