text: # Copyright 2018 The dm_control Authors.
  #
  # Licensed under the Apache License, Version 2.0 (the "License");
  # you may not use this file except in compliance with the License.
  # You may obtain a copy of the License at
  #
  #    http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS,
  # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or  implied.
  # See the License for the specific language governing permissions and
  # limitations under the License.
  # ============================================================================
  
  """Module defining the abstract robot class."""
  
  import abc
  
  from dm_control.composer import entity
  import numpy as np
  
  DOWN_QUATERNION = np.array([0., 0.70710678118, 0.70710678118, 0.])
  
  
  class Robot(entity.Entity, metaclass=abc.ABCMeta):
    """The abstract base class for robots."""
  
    @abc.abstractproperty
    def actuators(self):
      """Returns the actuator elements of the robot."""
      raise NotImplementedError
  