Thanks. Just for google-completeness, I still don't get it. I did get
it out, due to some unknown combination of pushing on the latch and
lifting at various places on the seat. But, even with it out, I'm not
sure. There is a divot where you say, which looks meant to accept the
screwdriver blade. And, without the seat in the picture, if I press on
that divot, the clip detaches cleanly from the floor bracket. Ha, I
thought. But, after I put the seat back in, I tried pressing there
again ... and got nothing.
So, I got it out. If I had to, I think I could probably do it again. I
don't know how.