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I recently asked a question on Math.SE and got an answer. However, I wanted some clarification so I wanted to add a comment. However, I am unable to use the @username feature to make sure that the answerer gets to know that I had requested some clarification. I have tried to edit and even post a new comment. I type @username and then my comment. However, when I press "Add Comment" Button, the @username simply disappears and only the main comment body appears in the comment.
Could someone explain what is going on and how can I notify the answerer?