I downloaded the source for "nethack", and it suggests I create a user and group named "games". But when I click on (gear symbol) -> System settings -> User accounts from the menu bar, Ubuntu will not allow me to create a user account named "games". When I type that name in, the "Create" button grays out. Yet the same window clearly shows me that my name is the only user account that already exists on the machine. What is the problem and how do I get around it?