I also would not object to that.
It still would be preferable not to have to jump forward every time you watch the demo though - I guess it depends on how often you are planning to watch it.
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The situation that I was thinking of is trying to optimise a room that has a number of distinct, non-interacting sections. I'd prefer to optimise each part separately, then just combine them at the end.
So my preferred method would be:
* Do the first section until I've got a pretty good score for that bit
* F4 to save the demo to that point
* Hit a checkpoint
* Do the second section until I've got a pretty good score for it.
* F4 to save the demo, then trim the demo, so it's just the second part
etc for each part.
* Watch each section so I can remember the best method.
* Do the room using the best bits. (without checkpoints)
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Heh, it's really just me being lazy and I can't envision a nice way to implement trimming - maybe watch the demo to a certain point, then f3 to cut before the current place, f4 to cut after it?