code: public PlanNode findAtOrBelow( Traversal order,
                                     Type typeToFind ) {
          return findAtOrBelow(order, EnumSet.of(typeToFind));
      }
docs: Find the first node with the specified type that are at or below this node.
  
  @param order the order to traverse; may not be null
  @param typeToFind the type of node to find; may not be null
  @return the first node that is at or below this node that has the supplied type; or null if there is no such node