code: def mark_dead(self, proxy, _time=None):
          
          if proxy not in self.proxies:
              logger.warn("Proxy <%s> was not found in proxies list" % proxy)
              return
  
          if proxy in self.good:
              logger.debug("GOOD proxy became DEAD: <%s>" % proxy)
          else:
              logger.debug("Proxy <%s> is DEAD" % proxy)
  
          self.unchecked.discard(proxy)
          self.good.discard(proxy)
          self.dead.add(proxy)
  
          now = _time or time.time()
          state = self.proxies[proxy]
          state.backoff_time = self.backoff(state.failed_attempts)
          state.next_check = now + state.backoff_time
          state.failed_attempts += 1
docs: Mark a proxy as dead