> Using \PackageError {package} {A traditional error} \@ehc:
>
> ! Package package Error: A traditional error.
>
> See the package package documentation for explanation.
> Type H <return> for immediate help.
> ...
>
> l.45 @ehc\endcsname}
>
> ?
>
> We still have the TeX part starting with the number, but no code line
> directly underneath the test.
Ah, yes. I was confusing myself.
I've never looked at how this is done internally before, but I'll add
it to the list.