[metapost] Type of loop index in for...within

[Nicola]

Hi,
it seems that I cannot find this explicitly stated anywhere: is the type 
of the loop index in a for... within loop always 'picture'? In other 
words, does the following *never* raise an error?

for i within currenpicture:
   if not picture i: errmessage "tsk tsk"; fi;
endfor;

Thanks,
Nicola