If you define 0,0 in co as a list, that should work:
(setq co (list 0 0))
of
(setq co '(0 0))
The difference being '( makes the string fixed so what follows is exactly as it is used, list( allows you to calculate the list items or refer to other variables
(setq String1 ("Hello") )
(setq string2 ("World") )
(setq mylist (list String1 String2)) -> is seen as (Hello World)
(setq mylist '(Strng1 String2)) -> is seen as (String1 String2) (though might throw up an error wanting "" around the text strings)