Ruby has an interesting property with arrays.  If I have an array [1, 2, 3] and I subtract an array [2, 3] then the result will be [1].  Since the set is an enumerable it can be converted into an array.  So you could also do something like this:

```ruby
filtered - filter_set.to_a
```

The way you did it is fine.  Just a fun piece of ruby knowledge.