Action when a comment has been replied to I have a unique case where we need to do something, namely a behavior where a user Comments on a node, and when a new comment is made in response to him, we need to "alert" the user in a variety of ways. Right now, lets say we need to send him an email.

I'm not sure of the best approach to implement this logic, but it looks like it may be possible to extend the Rules module to do so. What would this logic look like? Are there other approaches that can be used here?
I've got something similar in setting up a forum, where all posts on a topic are really just comments. We've set it up to allow people to "subscribe" to the topic via the Flag module and then have a Rule set up to notify all subscribers after a new comment is added. Here's the Rule (from /admin/config/workflow/rules )



And here's how it looks when you click "Edit" in the screen above:



So basically, I've set up an event for a comment being added to a forum topic, then under actions, fetch all the users who have flagged that node and loop through, sending them an email (which can be customized with tokens based on the newly added comment). You can automatically flag a user to a node when they create it or comment on it.