Another attempt after my sloppy 48ccdf322d.
@Infinisil, thanks again, reverted in 4794aa5de2 and explained my mistakes in 48ccdf322d (commitcomment-29678643). I start with their work and provide this proof of this commit's correctness:
```nix
(lib.fixedPoints.extends (lib.flip g) f) # now
((f: rattrs: self: let super = rattrs self; in super // f self super) (lib.flip g) f) # inline extends
(self: let super = f self; in super // (lib.flip g) self super) # beta reduce
(self: let super = f self; in super // g super self) # beta reduce
(self_: let super = f self_; in super // g super self_) # alpha rename
(self_: let super = f self_; in super // g super self_) # original, same
```
Eventually we might harmonize `overrideScope`'s `g` parameter with the general pattern, but I leave that breaking change as a separate step. Best not to refactor and break at once, and at least the abstractions make the oddity clearer.
3cf43547f4