issue on smart form

hi friends,

i want some help on smart forms

Requirment: when vendor has street address and PO Box address, use Street address.

if the main vendor has both a street address and po box address the program should use the street address

each vendor has following fields.

1)street/house number or pobox

2)postel code

3)city

4)region

if the main vendor does not have a street address and only has a PO BOX address ,the program should use the po box address.

plz sent me some logic on that.....

plz help me

regards

madhu