So there was a recent blitz cup, which was so fucking cool, Can it be added as a feature to cf? where u challenge your friend to a duel, and it automatically picks problems you haven't solved and opens one after a problem is solved, finding unsolved problems is already in gyms/mashups so the same functionality can be used, the only thing that needs to be added is the challenge/duel system, which the duel system already exists(bcz of the blitz cup), so the only thing that needs to be added is a challenge system where u can challenge someone to a duel, and u specify min rate/max rate for the problems, or just input the ratings. a separate duel rating would be cool with random match making, but even if its too complex to implement, just being able to challenge your friends shouldn't be that impossible, please MikeMirzayanov