tokens: For example , Deep Deterministic Policy Gradients ( DDPG ) lillicrap2015continuous and Trust Region Policy Optimization ( TRPO ) schulman2015trust can solve several control challenges in the Mujoco simulation environmenttodorov2012mujoco.heess2017emergence show that a variant of the Proximal Policy Optimization ( PPO)schulman2017proximal can learn to control a very high degree of freedom humanoid robot in Mujoco , even in very challenging environments .