My point was &quot;even&quot; and &quot;filter&quot; can be built by someone, just to be fair about the whole readability. If &quot;even&quot; is already built into the language, then that&#x27;s a given.<p>Readability and language understanding are not separable. I for one have a hard time actually understand what the code does above (although I can guess it). But I am not sure how ^ translate into [true false false true true true false false false false ...] sequence because, I thought we were calculating oddity of 1000 numbers.<p>[1 2 3 4 5 6 7 8 9 10 ...]   ==&gt;  [T F T T F T F T T T ]<p>Perhaps the lack of understanding Clojure syntax to comprehend what the code is doing.