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I took this kata as an opportunity to check my understanding of a pratt parser (http://journal.stuffwithstuff.com/2011/03/19/pratt-parsers-expression-parsing-made-easy/).
You are absolutely right. I still have to remember more of the clojure idioms;-).
if-not or not= might be more idiomatic for your sq-in-rect. Better yet would be a when-not since your topmost if in sq-in-rect doesn't have a second branch.