It seems inefficient to instantiate a new instance of `ExclusionRules` for each example.  Instead, I think you can do this:

``` ruby
def prune(examples)
  if inclusions.standalone?
    exclusions = ExclusionRules.new
    examples.select { |e| !exclusions.include_example?(e) && include?(e) }
  else
    # ...
  end
end
```