code: public Set<InternalQName> getSubtypes(final InternalQName nodeTypeName)
     {
        Set<InternalQName> resultSet = new HashSet<InternalQName>();
        for (InternalQName ntName : nodeTypes.keySet())
        {
           if (getSupertypes(ntName).contains(nodeTypeName))
           {
              resultSet.add(ntName);
           }
        }
        return resultSet;
     }
docs: Returns all subtypes of this node type in the node type inheritance
  hierarchy.
  
  @param nodeTypeName
  @return