text: import gym
  
  from rlkit.envs.custom.backward_env import BackwardHalfCheetah,\
          BackwardHopper, BackwardWalker
  from rlkit.envs.custom.fast_pendulum_env import FastPendulum
  
  gym.envs.register(
      id='BackwardHalfCheetah-v2',
      entry_point='rlkit.envs.custom.backward_env:BackwardHalfCheetah',
  )
  
  gym.envs.register(
      id='BackwardHopper-v2',
      entry_point='rlkit.envs.custom.backward_env:BackwardHopper',
  )
  
  gym.envs.register(
      id='BackwardWalker-v2',
      entry_point='rlkit.envs.custom.backward_env:BackwardWalker',
  )
  
  gym.envs.register(
      id='FastPendulum-v0',
      entry_point='rlkit.envs.custom.fast_pendulum_env:FastPendulum',
  )
  
  CUSTOM_ENVS = ['BackwardHalfCheetah', 'BackwardHopper', 'BackwardWalker',
                 'FastPendulum']
  