We also extend the previous scenario by allowing collaborative filtering, where data of multiple users
is needed by the advertisers to compute their recommendations. So requests for permission need to be sent
to multiple users, and only if they all agree is the advertiser allowed to aggregate their data for advertising
purposes. To further protect users from abuse, we also require that the user decisions remain confidential
from the advertisers, i.e. the specific users who allowed (or did not allow) use of their data as requested by
the advertiser should not be revealed to the server/advertiser.