minor fixes

[bblfish]

Other things that occurred to me:

1. List is a good example of a non-commutative monad

2. You mention that Arity ≅ FinSetº. This is very close to opposite of Set it seems (see question on StackExchange), which make me wonder what the relation to boolean algebras may be...

3. Whenever the Powerset monad pops up, there are some mathematicians (and programmers) that come up and ask awkward questions, which can get developers quite worried. It may be worth addressing this here and a bit more carefully in this book as a footnote, or as an appendix or as a pointer to a good explanation elsewhere (perhaps @paoloperrone's book).

   • It came up in the Zulip thread Polynomial Functors and Powerset
   • The first question Joachim Kock asks Bart Jacobs on his recent talk on Polynomials is regarding the Powerset functor which has some properties that don't quite fit the others.
   • Also I don't think Spivak includes the Powerset functor in Poly (or at least not as far as I have come yet following the course or reading the book).

Note I am really interested in the Powerset functor as the Kleisli category of Powerset is equivalent to the Category Rel of Relations, which is the key category in the definition of the Semantic Web as described by Evan Patterson in Knowledge Representation in Bicategories of Relations.

[bblfish]

Note: the comments I left above are not part of the PR really. They are just additional thoughts I had reading the first few chapters, but I did not want to open an issue for them.

[bblfish]

I think the reason FP programmers find Set collection types (ie PowerSets) problematic is that there is often no serialisation order for them, meaning that different runs can have different results, creating indeterminacy. Here is an example of a Set of 4 tuples, for RDF DataSets https://rdf4j.org/javadoc/latest/org/eclipse/rdf4j/model/Model.html but one finds similar things in the cats scala library.