
Why was 6 afraid of 7? Because 7 8 9!
Given a string apply the following transformations:
If there is a 6 next to a 7 remove the 6 (6 is afraid of 7)
If the sequence "789" appears remove the 8 and the 9 (7 ate 9)
(If I'm not mistaken it doesn't matter what order you do the transformations in...