MAYBE

Problem:
 int(0(),0()) -> .(0(),nil())
 int(0(),s(y)) -> .(0(),int(s(0()),s(y)))
 int(s(x),0()) -> nil()
 int(s(x),s(y)) -> int_list(int(x,y))
 int_list(nil()) -> nil()
 int_list(.(x,y)) -> .(s(x),int_list(y))

Proof:
 Open