148 § 156. Consent of Parties An international agreement may be modified, suspended, or terminated by the consent of the parties, except that when an international agreement confers a right, as indicated in § 139, upon a state not a party to the agreement, the consent of that state is required for the modification, suspension, or termination of the right, if either (a) the agreement provides for acceptance of the right and it has been accepted, or (b) there is no such provision but the state has changed its position in reliance upon the continuing existence of the right and its modification, suspension, or termination would be a substantial detriment to the state.