Subrule · Chapter 7
cr:7.3.2b
A card cannot be declared if (A) it does not have the defense property (0 is a value), (B) it is already a defending card on a chain link, or (C) it would make the current set of declared cards illegal to become defending.
Example: If an attack has overpower (can’t be defended by more than one action card) and is already defended by an action card, an action card cannot be declared because it cannot become a defending card.