code: def trees_by_issn(self, issn):
          
          return set(
              self.issn_db.get(issn, OOSet()).keys()
          )
docs: Search trees by `issn`.
  
          Args:
              issn (str): :attr:`.Tree.issn` property of :class:`.Tree`.
  
          Returns:
              set: Set of matching :class:`Tree` instances.