docs: def GetDefaultToken(token):
    """Returns the provided token or the default token.
  
    Args:
      token: A token or None.
  
    Raises:
      access_control.UnauthorizedAccess: no token was provided.
    """
    if token is None:
      token = default_token
  
    if not isinstance(token, access_control.ACLToken):
      raise access_control.UnauthorizedAccess(
          "Token is not properly specified. It should be an "
          "instance of grr.lib.access_control.ACLToken()")
  
    return token