code: def tick(self):
          
          for npc in self.npcs:
              self.move_entity(npc, *npc.towards(self.player))
          for entity1, entity2 in itertools.combinations(self.entities, 2):
              if (entity1.x, entity1.y) == (entity2.x, entity2.y):
                  if self.player in (entity1, entity2):
                      return 'you lost on turn %d' % self.turn
                  entity1.die()
                  entity2.die()
  
          if all(npc.speed == 0 for npc in self.npcs):
              return 'you won on turn %d' % self.turn
          self.turn += 1
          if self.turn % 20 == 0:
              self.player.speed = max(1, self.player.speed - 1)
              self.player.display = on_blue(green(bold(unicode_str(self.player.speed))))
docs: Returns a message to be displayed if game is over, else None