I know that using @username in a comment I can ping people who were "engaged in the post", which includes:
The poster (IIRC he is pinged even without @username ).
Someone who wrote some of the comments.
What about if some user edited the post, but did not post any comments. There are situations where it would be good to get his attention. (In particular if there is need to explain to the user that his edit was - for some reason - redundant, unwanted or inappropriate. Or if someone want to explain, how the user could have improved his edit.) Would @username in a comment work in such situation?