the fact that one subscription was added but forgotten in the removal process is a smell. Let's go one step further and refactor this to:

```ruby
@_subscribers = []
# ...
@_subscribers << ActiveSupport::Notifications.subscribe("A")
@_subscribers << ActiveSupport::Notifications.subscribe("B")
@_subscribers << ActiveSupport::Notifications.subscribe("C")

# loop over @_subscribers to remove them.
```

This should be easier to keep in sync.