I think the Add function is correct. 

Sub is only correct if Add is correct. 
This is draft 1
No need to change this now, but in the future, you can also do `t.Errorf` to print the message and cause the tests to fail (this won't cause them to terminate immediately though), so the rest of the error messages will be printed as well.

`t.Fatalf` is the equivalent of `t.Errorf` + `t.FailNow`.