code: func receiversToEphemeralKeyCreator(receivers []BoxPublicKey) (EphemeralKeyCreator, error) {
  	if len(receivers) == 0 {
  		return nil, ErrBadReceivers
  	}
  	return receivers[0], nil
  }
docs: // receiversToEphemeralKeyCreator retrieves the EphemeralKeyCreator
  // from the first receiver; this is to preserve API behavior.