code: protected int _level(int identity)
    {
      while (identity>=m_size)
      {
        boolean isMore = nextNode();
        if (!isMore && identity >= m_size)
          return NULL;
      }
  
      int i=0;
      while(NULL != (identity=_parent(identity)))
        ++i;
      return i;
    }
docs: Get the level in the tree for the given node identity.
  
  @param identity The node identity.
  
  @return The tree level, or DTM.NULL.