```# #trace tz;;
t1 is now traced.
# tz ([], ['a'; 'b'; 'S'; 'd']);;
tz <-- ([], ['a'; 'b'; 'S'; 'd'])
tz <-- (['a'], ['b'; 'S'; 'd'])         (* Pull zipper *)
tz <-- (['b'; 'a'], ['S'; 'd'])         (* Pull zipper *)
tz <-- (['b'; 'a'; 'b'; 'a'], ['d'])    (* Special step *)
tz <-- (['d'; 'b'; 'a'; 'b'; 'a'], [])  (* Pull zipper *)
tz --> ['a'; 'b'; 'a'; 'b'; 'd']        (* Output reversed *)
tz --> ['a'; 'b'; 'a'; 'b'; 'd']
tz --> ['a'; 'b'; 'a'; 'b'; 'd']
tz --> ['a'; 'b'; 'a'; 'b'; 'd']
tz --> ['a'; 'b'; 'a'; 'b'; 'd']
- : char list = ['a'; 'b'; 'a'; 'b'; 'd']
```
The nice thing about computations involving lists is that it's so easy to visualize them as a data structure. Eventually, we want to get to a place where we can talk about more abstract computations. In order to get there, we'll first do the exact same thing we just did with concrete zipper using procedures. Think of a list as a procedural recipe: `['a'; 'b'; 'S'; 'd']` is the result of the computation `'a'::('b'::('S'::('d'::[])))` (or, in our old style, `make_list 'a' (make_list 'b' (make_list 'S' (make_list 'd' empty)))`). The recipe for constructing the list goes like this:
```(0)  Start with the empty list []
(1)  make a new list whose first element is 'd' and whose tail is the list constructed in step (0)
(2)  make a new list whose first element is 'S' and whose tail is the list constructed in step (1)
-----------------------------------------
(3)  make a new list whose first element is 'b' and whose tail is the list constructed in step (2)
(4)  make a new list whose first element is 'a' and whose tail is the list constructed in step (3)
```