[web2py] db and variables

[Rick]

Hi,

I'm trying to write loops in db.py:

letters = ['a', 'b', 'c', 'd']
for i in range(len(letters)):
        for j in range(len(letters)):

                db.define_table('connection'+letters[i]+letters[j],
                        SQLField('smthng', db.???letters[i]))

What should I use instead of ???  to make the computer understand that
as db.a when i=0 and j=0?

Thanks in advance for help!