Robot Name

Manage robot factory settings.

When a robot comes off the factory floor, it has no name.

The first time you turn on a robot, a random name is generated in the format
of two uppercase letters followed by three digits, such as RX837 or BC811.

Every once in a while we need to reset a robot to its factory settings,
which means that its name gets wiped. The next time you ask, that robot will
respond with a new random name.

The names must be random: they should not follow a predictable sequence.
Using random names means a risk of collisions. Your solution must ensure that
every existing robot has a unique name.