docs: @Override
      public int getIndexOfChild(Object parentObj, Object childObj) {
          if (debugger == null) {
              return -1;
          }
          VariableNode parent = (VariableNode) parentObj;
          VariableNode child = (VariableNode) childObj;
          VariableNode[] children = children(parent);
          for (int i = 0; i != children.length; i++) {
              if (children[i] == child) {
                  return i;
              }
          }
          return -1;
      }