old_contents: from chainerrl.agents.a3c import A3C  # NOQA
  from chainerrl.agents.acer import ACER  # NOQA
  from chainerrl.agents.al import AL  # NOQA
  from chainerrl.agents.ddpg import DDPG  # NOQA
  from chainerrl.agents.double_dqn import DoubleDQN  # NOQA
  from chainerrl.agents.double_pal import DoublePAL  # NOQA
  from chainerrl.agents.dpp import DPP  # NOQA
  from chainerrl.agents.dqn import DQN  # NOQA
  from chainerrl.agents.nsq import NSQ  # NOQA
  from chainerrl.agents.pal import PAL  # NOQA
  from chainerrl.agents.pcl import PCL  # NOQA
  from chainerrl.agents.pgt import PGT  # NOQA
  from chainerrl.agents.ppo import PPO  # NOQA
  from chainerrl.agents.reinforce import REINFORCE  # NOQA
  from chainerrl.agents.residual_dqn import ResidualDQN  # NOQA
  from chainerrl.agents.sarsa import SARSA  # NOQA
  from chainerrl.agents.trpo import TRPO  # NOQA
  
new_contents: from chainerrl.agents.a3c import A3C  # NOQA
  from chainerrl.agents.acer import ACER  # NOQA
  from chainerrl.agents.al import AL  # NOQA
  from chainerrl.agents.c51 import C51  # NOQA
  from chainerrl.agents.ddpg import DDPG  # NOQA
  from chainerrl.agents.double_dqn import DoubleDQN  # NOQA
  from chainerrl.agents.double_pal import DoublePAL  # NOQA
  from chainerrl.agents.dpp import DPP  # NOQA
  from chainerrl.agents.dqn import DQN  # NOQA
  from chainerrl.agents.nsq import NSQ  # NOQA
  from chainerrl.agents.pal import PAL  # NOQA
  from chainerrl.agents.pcl import PCL  # NOQA
  from chainerrl.agents.pgt import PGT  # NOQA
  from chainerrl.agents.ppo import PPO  # NOQA
  from chainerrl.agents.reinforce import REINFORCE  # NOQA
  from chainerrl.agents.residual_dqn import ResidualDQN  # NOQA
  from chainerrl.agents.sarsa import SARSA  # NOQA
  from chainerrl.agents.trpo import TRPO  # NOQA
  
subject: Add C51 to __init__.py