code: def truncate_seq_pair(tokens_a, tokens_b, max_num_tokens, rng):
      
      while True:
          total_length = len(tokens_a) + len(tokens_b)
          if total_length <= max_num_tokens:
              break
  
          trunc_tokens = tokens_a if len(tokens_a) > len(tokens_b) else tokens_b
          assert len(trunc_tokens) >= 1
  
          # We want to sometimes truncate from the front and sometimes from the
          # back to add more randomness and avoid biases.
          if rng.random() < 0.5:
              del trunc_tokens[0]
          else:
              trunc_tokens.pop()
docs: Truncates a pair of sequences to a maximum sequence length.