True!
So I’m guessing #38 is what you had in mind:*COMMON NORMS
Can. 35 A singular administrative act, whether it is a decree, a precept, or a rescript, can be issued by one who possesses executive power within the limits of that person’s competence, without prejudice to the prescript of ⇒ can. 76, §1.
Can. 36 §1. An administrative act must be understood according to the proper meaning of the words and the common manner of speaking. In a case of doubt, those which refer to litigation, pertain to threatening or inflicting penalties, restrict the rights of a person, injure the acquired rights of others, or are contrary to a law which benefits private persons are subject to a strict interpretation; all others are subject to a broad interpretation.
§2. An administrative act must not be extended to other cases besides those expressed.
Can. 37 An administrative act which regards the external forum must be put in writing.
Furthermore, if it is given in commissariat form, the act of its execution must be put in writing.
**Can. 38 An administrative act, even if it is a rescript given motu proprio, lacks effect insofar as it injures the acquired right of another or is contrary to a law or approved custom, unless the competent authority has expressly added a derogating clause.
Can. 39 Conditions in an administrative act are considered added for validity only when they are expressed by the particles if (si), unless (nisi), or provided that (dummodo).
Can. 40 The executor of any administrative act invalidly carries out his or her function before receiving the relevant letter and verifying its authenticity and integrity, unless previous notice of the letter had been communicated to the executor by authority of the one who issued the act.
And so since excommunication is a pastoral (not a function of the Pope’s
teaching office), you would say that canon 38 prevents a Pope from making an excommunication against someone who was conformed to prior custom with the exception of a “derogating clause”----what might that be?