After you finish this exercise, you will be given the password allowing you to make your computer
do much of the "busy" work of derivations.
The password is '77'! It lets you do the next exercise.
There you don't have to type in sentences. Just type in the justification
and a '/'. The computer can usually figure out what sentence you meant.
But there's more. Read to the end of tutorial 5.6 for
the details on how you can do derivations with less work.