[osa1]

I found writing some complex data structures is great way to learn basics of language. And then you can iterate and improve your data structure to make the code more idiomatic and/or faster.

I grasped Common Lisp with writing `Hash Array Mapped Trie` data structure(one of the data structures lie within some of Clojure's persistent data structures). It helped me understand both the particular data structure and the language.