Why haven't they made it a rule that if a creature's power or toughness are set by an ability (the creature is a */*, */whatever or whatever/*), and then that creature loses all abilities, it because a 0/0, 0/whatever or whatever/0? It would clear up ambiguity with
Ground Seal and
Yixlid Jailer, and allow "target creature loses all abilities" language without having to specify a power or toughness.
What interaction is bothering you? For Example, are you wondering if
Tarmogoyf's P/T in the GY when
Yixlid Jailer is on the Battlefield is set to 0/0?
Current Rules, that I could find so far, that apply are:
CR wrote:
112.12. An effect that sets an object’s characteristic, or simply states a quality of that object, is different from an ability granted by an effect. When an object “gains” or “has” an ability, that ability can be removed by another effect. If an effect defines a characteristic of the object (“[permanent] is [characteristic value]”), it’s not granting an ability. (See rule 604.3.) Similarly, if an effect states a quality of that object (“[creature] can’t be blocked,” for example), it’s neither granting an ability nor setting a characteristic.
CR wrote:
208.2. Rather than a fixed number, some creature cards have power and/or toughness that includes a star (*).
208.2a The card may have a characteristic-defining ability that sets its power and/or toughness according to some stated condition. (See rule 604.3.) Such an ability is worded “[This creature’s] [power or toughness] is equal to . . .” or “[This creature’s] power and toughness are each equal to . . .” This ability functions everywhere, even outside the game. If the ability needs to use a number that can’t be determined, including inside a calculation, use 0 instead of that number.
208.2b The card may have a static ability that creates a replacement effect that sets the creature’s power and toughness to one of a number of specific values as it enters the battlefield or is turned face up. (See rule 614, “Replacement Effects.”) Such an ability is worded “As [this creature] enters the battlefield . . . ,” “As [this creature] is turned face up . . . ,” or “[This creature] enters the battlefield as . . .” and lists two or more specific power and toughness values (and may also list additional characteristics). The characteristics chosen or determined with these effects affect the creature’s copiable values. (See rule 706.2.) While the card isn’t on the battlefield, its power and toughness are each considered to be 0.
CR wrote:
604.3. Some static abilities are characteristic-defining abilities. A characteristic-defining ability conveys information about an object’s characteristics that would normally be found elsewhere on that object (such as in its mana cost, type line, or power/toughness box) or overrides information found elsewhere on that object. Characteristic-defining abilities function in all zones. They also function outside the game.
604.3a A static ability is a characteristic-defining ability if it meets the following criteria: (1) It defines an object’s colors, subtypes, power, or toughness; (2) it is printed on the card it affects, it was granted to the token it affects by the effect that created the token, or it was acquired by the object it affects as the result of a copy effect or text-changing effect; (3) it does not directly affect the characteristics of any other objects; (4) it is not an ability that an object grants to itself; and (5) it does not set the values of such characteristics only if certain conditions are met.
Gatherer Errata wrote:
From the
Yixlid Jailer Page: If Mistform Ultimus is in the graveyard, the Ultimus will lose its ability that says "Mistform Ultimus is every creature type," but it will still *be* all creature types. The way continuous effects work, Mistform Ultimus's type-changing ability is applied before Yixlid Jailer's ability removes it.
CR 112 implies a characteristic defining effect can't be removed, the Errata implies similarly that while the ability itself becomes removed (as defined in 604- Characteristic-defining Abilities) the effect will have already have happened and will still be in effect.
I'm not sure what a higher ruling would be, but I expect that the P/T would be stuck at the last defined state before the ability was lost. Is there a reason it should become 0/0?
Hope that helps.