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, `makelist a (makelist b (makelist S (makelist c empty)))`).
+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:
<pre>
context, a continuation is a function of type `char list -> char
list`. For instance, the continuation corresponding to the portion of
the recipe below the horizontal line is the function `fun (tail:char
-list) -> a::(b::tail)`.
+list) -> 'a'::('b'::tail)`.
This means that we can now represent the unzipped part of our
-zipper--the part we've already unzipped--as a continuation: a function
+zipper---the part we've already unzipped---as a continuation: a function
describing how to finish building the list. We'll write a new
function, `tc` (for task with continuations), that will take an input
list (not a zipper!) and a continuation and return a processed list.