code: def update_ttl(self, ttl):
          
  
          if len(self) == 0:
              self.ttl = ttl
          elif ttl < self.ttl:
              self.ttl = ttl
docs: Set the TTL of the rdataset to be the lesser of the set's current
          TTL or the specified TTL.  If the set contains no rdatas, set the TTL
          to the specified TTL.
          @param ttl: The TTL
          @type ttl: int