Thanks to all the replies!
It seems that there are ways to solve the exercise if I use constructs that are found in later chapters of the book. Sean could be right in that some of the exercises are meant to be difficult to solve and they prepare you for later chapters.
As a programmer, I was unhappy with my answers to some of the earlier exercises because they were clearly inefficient and/or inelegant. This kept me reading the book, knowing the answers would present themselves.
On more than one occasion I would fret for a few hours over an exercise and then a day or two later learn something new two chapters later which made the exercise much, much easier. Sometimes trivial.
As an aside, I kept all of the exercises in revision control. So I can look back at what I first wrote and my later changes. A habit I plan to keep as I move on to other programming texts and languages.
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