def define_actions(action):
    """Given an action string, returns a list of corresponding actions.

  Args
    action: String. either "all" or one of the h36m actions
  Returns
    actions: List of strings. Actions to use.
  Raises
    ValueError: if the action is not a valid action in Human 3.6M
  """
    actions = [
        "Directions", "Discussion", "Eating", "Greeting", "Phoning", "Photo", "Posing", "Purchases", "Sitting",
        "SittingDown", "Smoking", "Waiting", "WalkDog", "Walking", "WalkTogether"
    ]

    if action == "All" or action == "all":
        return actions

    if action not in actions:
        raise ValueError("Unrecognized action: %s" % action)

    return [action]