docs: public void addIntermediary(AbstractPlanNode node) {
  
          // transfer this node's children to node
          Iterator<AbstractPlanNode> it = m_children.iterator();
          while (it.hasNext()) {
              AbstractPlanNode child = it.next();
              it.remove();                          // remove this.child from m_children
              assert child.getParentCount() == 1;
              child.clearParents();                 // and reset child's parents list
              node.addAndLinkChild(child);          // set node.child and child.parent
          }
  
          // and add node to this node's children
          assert(m_children.size() == 0);
          addAndLinkChild(node);
      }