On 13/09/2008, at 3:51 PM, Joseph Wright wrote:

> 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.