Crosslink requires modifying the two subprotocols objective validity rules, since they each refer to the other subprotocol's state.
However, the fork-choice rules of subprotocols are preserved.
It's worth documenting this higher-level delineation of changes imposed on the subprotocols somewhere in the book, because it could be missed by many readers when reviewing a lower level / more concrete specification of the Crosslink mechanism.
Suggested Improvement
Crosslink requires modifying the two subprotocols objective validity rules, since they each refer to the other subprotocol's state.
However, the fork-choice rules of subprotocols are preserved.
It's worth documenting this higher-level delineation of changes imposed on the subprotocols somewhere in the book, because it could be missed by many readers when reviewing a lower level / more concrete specification of the Crosslink mechanism.