The Scheme Programming Language, Skim carefully (if that's not an oxymoron) through chapters 1-2. There's a lot of material here to absorb at once; your goal is to familiarize yourself with what's here so you'll remember to look for it later when you have questions about it.
Reading: Polymorphism. This article is less coherent than it should be. Read it (the opening section in particular) with a critical eye so we can talk about what polymorphism actually is, but also how the article could be improved.
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