Race Registry
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Context: a system for organizing a 5K running race.

When runners register for a race, they provide their name, email address and
their speed category. A speed category indicates how quickly they estimate that
they can finish the race. This allows organizers to start the runners in groups
of roughly equivalent running speed so that faster runners aren't stuck behind
slower runners. The possible speed categories are: under 20 minutes, under 30
minutes, under 40 minutes, and 40 minutes or over. We need to be able get a list
of runners in a given speed category. We also need to be able to look up a
runner to find their speed category. Finally, a runner should be able to change
their email address and speed category, or withdraw from the race entirely.