The joke is that, if you don't need to check if the input is negative because it is not a natural number, you can also choose a definition of the natural number without zero to avoid special casing it at all.
This book builds up a mathematics in which multiplying two negative numbers gives you a negative, not a positive. The results are very interesting, particularly this: There are no complex numbers. The root of -1 is -1.
This is another example of something you never think to question, you always assume is "just the natural way", but which was a somewhat arbitrary choice and can be changed.
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