thanks

That's some example code.

(ns udacity.inferential
    (:require (incanter [core :refer :all]
                        [stats :refer :all]
                        [charts :refer :all]
                        [datasets :refer :all]
                        [io :refer :all])
               [clojure.math.combinatorics :as combo]))

(defn pop-sd [data]
  (let [m (mean data)]
    (->> data
       (map (comp
             #(*' % %)
             #(-' % m)))
       mean
       sqrt)))

You don't even need to think about it just use paredit or smartparens. Any clojure editor or IDE supports it, Emacs, LightTable, Counterclockwise, Cursive. Because it's automatic, you'll feel the code flow naturally. For instance Ctrl-right on Emacs will do this for you:
()[1 2 3] -> ([1 2 3]) Ctrl-left will do the inverse. Besides that a nice IDE will have rainbowparens so you can visually gauge the depth of the s-expr.

Also I don't understand how your function tests triangularity. Could you point me to a book or provide a proof that it's true for all triangular numbers. It's fascinating really.

S-expressions close on the last word, just look at clojure's sourcecode to see what I mean.

S-expressions close on the last word just look at clojure's sourcecode.

(S-expressions
close
on the
last word)
(Don't
(do
this it looks silly lisp isn't algol
)
)