code: public boolean isBelow( PlanNode possibleAncestor ) {
          PlanNode node = this;
          while (node != null) {
              if (node == possibleAncestor) return true;
              node = node.getParent();
          }
          return false;
      }
docs: Determine if the supplied node is an ancestor of this node.
  
  @param possibleAncestor the node that is to be determined if it is an ancestor
  @return true if the supplied node is indeed an ancestor, or false if it is not an ancestor