Aha, this one i suppose (your blog does not have a browseable index, although it does have search): https://blog.janetacarr.com/clojure-transducers-your-composa...

Personally, i would say that a blog post which starts "We can think of a transducer as a context-independent transformation composed of, say, many reducers" and then starts adding parentheses is not really demystifying. But perhaps i am not the target audience.