concreteness and understandability of the zipper provides a way of
understanding an equivalent treatment using continuations.
-Let's work with lists of `char`s for a change. To maximize readability, we'll
-indulge in an abbreviatory convention that "abSd" abbreviates the
-list `['a'; 'b'; 'S'; 'd']`.
+Let's work with lists of `char`s for a change. We'll sometimes write
+"abSd" as an abbreviation for
+`['a'; 'b'; 'S'; 'd']`.
We will set out to compute a deceptively simple-seeming **task: given a
string, replace each occurrence of 'S' in that string with a copy of