Keywords of C.
These are listed in [C:6.4.1]:
auto extern short while break float signed _Alignas case for sizeof _Alignof char goto static _Atomic const if struct _Bool continue inline switch _Complex default int typedef _Generic do long union _Imaginary double register unsigned _Noreturn else restrict void _Static_assert enum return volatile _Thread_local
They consist of all ASCII characters, and therefore they are directly representable as ACL2 strings.