I think what you wrote and what I wrote is equivalent, boolean-wise... in fact, I think all of the following are equivalent:
```
(cfg.stable -> cfg.stable && !cfg.testing)
<=>
# according to the nix manual, e1 -> e2 is equal to !e1 || e2
!cfg.stable || (cfg.stable && !cfg.testing) 
<=>
!cfg.stable || !cfg.testing
<=>
# !e1 || e2 is equal to e1 -> e2 
cfg.stable -> !cfg.testing
```
But it doesn't matter very much, just ignore me :)