Add spec for Rule class
user
```ruby
require 'fast_spec_helper'
require 'app/models/rule'

describe Rule, '#violated?' do
  context 'when child class does not implement the method' do
    it 'raises an exception' do
      class TestRule < Rule; end
      rule = TestRule.new('test')

      expect { rule.violated? }.to raise_error(/Must implement #violated\?/)
    end
  end
end
```
assistant