where to add <multiple="multiple"> in profile.module (for a multiple 'list selection') ?

hi,

i just would like to make my 'list selection' of the profile fields multiple (for more than one choice). i guess all what it needs is to add somewhere the . i had a look around in the profile.module, but i am not quite sure if this the place to be... does somebody know?

thanks a lot in advance.

best,
mohamed