This rule can be used forwards to eliminate a double not from the front of a formula or backwards to add a double not onto the front of a goal formula.
To use it forwards:

1. select both an empty line and Not Not Elimination.
2. then select a NOT NOT formula line to eliminate (the double not from this line must be in the scope of the empty line).
3. the sub formula of the NOT NOT formula will then be added to your proof.

1. select both the goal line you wish to add a double not to and Not Not Elimination.
2. the goal formula with a double not on the front of it will then be added to your proof as a new goal.