I'm reading the paper “Collision-Resistant Hashing? Towards Making UOWHFs Practical” which states:
With an ACR hash function $F$ the key $K$ is announced and the adversary wins if she manages to any collision $M,M'$ for $f = F_K$. The points $M$ and $M'$ may depend arbitrarily on $K$.
Could you give me an example of messages $M$ and $M'$, so that they depend arbitrarily on $K$?