boolean isFrozen() {
   return baseGraph.isFrozen();
 }
 /**
  * Avoid that edges and nodes of the base graph are further modified. Necessary as hook for e.g.
  * ch graphs on top to initilize themself
  */
 public void freeze() {
   if (!baseGraph.isFrozen()) {
     baseGraph.freeze();
   }
 }