Given string 'a.bc.' -- replace each dot(.) with 0 or 1

How can i write this function simply?   (in Common Lisp)
-- Given a string  'a.bc.'   -- replace each dot(.)  with 0 or 1.
        -- So the value is a list of 4 strings:
                                  ('a0bc0'  'a0bc1'  'a1bc0'  'a1bc1')
-- The order is not important.
             If the string has 3 dots, the value is a list of length 8.
If the program is going to be simpler,
                       pls use, e.g.   (a $ b c $)  rather than  'a.bc.'