[bug#29745] Why should build phases not return unspecified values?

[Arun Isaac]

> Concretely, we can:
>
>   1. Encourage use of ‘invoke’ when reviewing or writing new package
>      definitions;
>
>   2. Gradually migrate packages (we can do a bit of that in
>      ‘core-updates’, though we won’t do full rebuilds at this stage).
>
> How does that sound?

Sounds good. I agree to all the above.