code: public long containedChildren(final ITreeData paramNodeX, final ITreeData paramNodeY) throws TTIOException {
          assert paramNodeX != null;
          assert paramNodeY != null;
          long retVal = 0;
  
          mRtxOld.moveTo(paramNodeX.getDataKey());
          for (final AbsAxis axis = new DescendantAxis(mRtxOld, true); axis.hasNext(); axis.next()) {
              retVal += mIsInSubtree.get(paramNodeY, partner(mRtxOld.getNode())) ? 1 : 0;
          }
  
          return retVal;
      }
docs: Counts the number of child nodes in the subtrees of x and y that are also
  in the matching.
  
  @param paramNodeX
  first subtree root node
  @param paramNodeY
  second subtree root node
  @return number of children which have been matched
  @throws TTIOException