ruby solution. Uses [] not () as per rule #1.

As per rules 1 and 8:

> 1. You are encouraged to break the rules, cleverly.
> 8. If you have a solution that is close, but does not meet these rules, submit it anyway. A close and interesting solution is better than no solution.

```
$ ./solutions/ruby-pda/goal.rb
gal
goal
gooal
goooal
```