Update README.md

2019/README.md

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 ## MB’s Advent of Code tips
 
+* The problems are often designed around a particular computer-y abstraction. If you notice what the abstraction is, and then find the closest analog in Racket, the solution tends to come together quickly. Otherwise, you can spend a lot of time reinventing the wheel.
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 * Complex numbers are a nice way of modeling two-dimensional positions.
 
-* Use lists whenever feasible, because there are many useful list functions in the Racket library that don’t have vector equivalents.
+* Use lists whenever feasible, because there are many useful list functions in the Racket library that don’t have vector equivalents. In particular, [these list functions](https://docs.racket-lang.org/reference/pairs.html?q=racket%2Flist#%28part._.Additional_.List_.Functions_and_.Synonyms%29) are very useful, especially `argmin` and `argmax`.
 
-* In particular, [these list functions](https://docs.racket-lang.org/reference/pairs.html?q=racket%2Flist#%28part._.Additional_.List_.Functions_and_.Synonyms%29) are very useful.
+* Vectors are better than lists in situations where you need random access to members.
 
 * `eq?` is the fastest equality check, but it only works for symbols and fixnums (therefore, use more symbols and fixnums so you can use `eq?`!)
 
 * `match` is fantastic.
 
-* Association lists (= lists of pairs) can be used with the dictionary forms (like `dict-ref` and `in-dict`)
+* Association lists (= lists of pairs) are underrated. They’re compatible with all the usual list functions, of course, but also dictionary forms (like `dict-ref` and `in-dict`).
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+* The `graph` library can be helpful for graph-based problems.
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+* It’s good to know about sets and mutable pairs.
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+* Also the fancier `for` iterators, like `for/first` and `for/or`.
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-* The `graph` library can be useful for graph-based problems.
 
 ## My solutions
 
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 * I don’t optimize for speed.
 
 * I like doing the Advent of Code problems because it forces me to use parts of Racket that I don’t ordinarily use. So I treat it as a chance to expand my awareness of the Racketverse.
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+* I’m unlikely to finish every problem. Judging by past years, there is a point where the problems get sufficiently complex that I’d rather put that time into improving my other Racket projects :metal: