Another follow-up from yesterday's doc reviews

[Robert Goldman]

On 9/22/16 Sep 22 -1:56 PM, Stelian Ionescu wrote:
>> The comments in the file say that we *must* use load-asdf to load a
>> .asd file, but not *WHY* we must do so.
>> 
>> Why is this necessary?
> 
> Because asdf:load-asdf sets up the correct evaluation environment,
> including setting the correct *package*.
> 

OK, again, if this is required, why don't we ENFORCE it?

It would be trivial to bind a special variable around the body of
LOAD-ASD, and put something in DEFSYSTEM that will raise an INTELLIGIBLE
error if that variable is not so bound

(error "Do not load an ASDF system definition outside of ASDF:LOAD-ASD")

Otherwise, we are just encouraging programmers to shoot themselves in
the foot.

I will put in place such a check.