question about the semantics of adding a gas change

[Dirk Hohndel]

It feels like we talked about this before. But after playing around with
both the planner and the dive editor for a while I'm not sure I like what
we do (or, frankly, fully understand what we do).

So let's enter "add dive" mode. A very basic profile shows up.
Now add a second tank with pure oxygen. 
Click on the handle at the beginning of the safety stop and try to switch to pure oxygen
(right click, context menu offers AIR and OXYGEN, select OXYGEN)
No error, no nothing, doesn't work.

OK, maybe there's some magic to keep my PO2 in a safe range, but no message?

Next attempt, change the tank to EAN80 

Again, right click on the handle at the beginning of the safety stop (at
5m). Pick EAN80

Now the ascent from depth to 5m is using EAN80 and there (at the node I
clicked on) we switch back to air. Not exactly intuitive. And of course
this means that I cannot actually end the dive on a different gas than I
started with. 

But quite frankly, why DOES it change the previous segment.
If I click at a spot in the profile in normal view mode and add a tank
change then the gas from that point on changes. That seems to be the
logical thing to do. Yet here we change the gas used during the segment up
the point we are on.

Can someone explain why this makes sense?

/D