code: def new_tile(self, num=1):
          
          for _ in range(num):                
              if random.random() > .5: 
                  new_tile = self.pieces[0]
              else:
                  new_tile = self.pieces[1]
              
              # check for game over
              blanks = self.count_blank_positions()
              
              if blanks == 0:
                  print ("GAME OVER")
              else:
                  res = self.find_random_blank_cell()
                  row = res[0]
                  col = res[1]
                  self.set_tile(row, col, new_tile)
docs: Create a new tile in a randomly selected empty 
          square.  The tile should be 2 90% of the time and
          4 10% of the time.