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Could you do 'if last post'?
There are always edge cases... what does "last post" mean? Last visible? Last if nothing is merged into the conversation? Last only when not counting things in moderation queues?
Time is the best way to do it... you have a predictable window in which errors can be corrected, afterwards the fact that you made a comment is part of the historical record but we'd let you change what you said (a history is kept for legal reasons).
vBulletin functionality used to be that you could delete your posts if the posts were made within 15 minutes, and after that you had to edit them.
The reason for that is to avoid breaking context and tricking people, i.e. to coax someone into saying something, nuke your post, and have their comment change context into something humiliating or offensive.
They allowed quick deletions as sometimes comments were made in error, in the wrong place, etc.
Presently we only allow the edits, but we are interested in returning a time based feature that would permit proper deletes if done within a short amount of time.