text: from abc import ABC, abstractmethod
  
  
  class GoalSamplingStrategy(ABC):
  
      @abstractmethod
      def __call__(self, episode_transitions, n_hindsight_goals):
          """Return a list of indices of transitions from which goals are generated.
  
          Hindsight goals are constructed from the achieved goal attribute of the 
          selected transitions."""
          pass
  
  
  