I disagree w/ this comment. The types of problems where it disallows using built-ins are typically those that have you re-implementing a function for purposes

I disagree w/ this comment. The types of problems where it disallows using built-ins are typically those that have you re-implementing a function for purposes of learning. The implementation of that function, in my experience, is still fairly idiomatic if you are following the right users. Particularly those functions that force you to build a lazy implementation. I found the exercises (and more importantly comparing to more experienced user's solutions) incredibly helpful not just for clojure but FP concepts generally.