Obsession! does not unlet v:this_session

[benknoble]

Since session tracking is stopped and the relevant file deleted, I contend that v:this_session should be unlet.

This is mostly because after running Obsession! the status indicator shows [S], as though there were (paused) tracking.

A different solution would be to remove the v:this_session part of the check from the status line flags, but I imagine part of the idea there was to keep the status line flag visible even when using non-Obsession session files.

[tpope]

Can't unlet it but could set it to an empty string. My thinking was that leaving it set would be useful in case you later want to recreate it, but I guess that's a pretty contrived use case.