JBU7ITG4WFYNXLK3KL6BTQ4DXP7NDRPBGKYWNJX3Z6FT4HIA63PQC
H6XH45OPVRRAAP2N4AK3DCHVVSMVQ6B3MQZYKU7YVCN4O2IJGQUQC
// I AM NOT DONE
assert_eq!(times_two(4), ???);
assert_eq!(times_two(4), 8);
// TODO replace unimplemented!() with an assert for `times_two(-4)` unimplemented!()
// TODO replace unimplemented!() with an assert for `times_two(-4)`
unimplemented!()
assert_eq!(times_two(-4), -8);