The Great Clade Race

Imagine a race through the woods with six runners (A-F). All the runners start at a single starting line but at various places the course forks, and runners are free to choose either path. The paths never converge again (once two runners pick different paths they can never meet up again). To help keep track of the race, each runner carries a card that gets stamped at check-in stations distributed at various places in the course. The following rules apply: