code: def collect_publications(self):
          
          pubs = list(self.sub_publications)
  
          for sub_tree in self.sub_trees:
              pubs.extend(sub_tree.collect_publications())
  
          return pubs
docs: Recursively collect list of all publications referenced in this
          tree and all sub-trees.
  
          Returns:
              list: List of UUID strings.