Sniffed this trick from the Incanter source. Here's a demo of using tree composition to calculate a 2-d Euclidean distance between two points.

```
(def x [1 2])
(def y [4 5])
(defn- tree-comp-each [root branch & leaves]
(apply root (map branch leaves)))
(defn euclidean-distance [a b]
{:pre [(= (count a) (count b))]}
(sqrt (apply tree-comp-each +
(fn [[x y]]
(pow (- x y) 2))
(map vector a b))))
```

I've only had exposures to tree traversal in the use of implementing searching and sorting algorithms. This trick here definitely widened my eyes to the wonders of functional programming. It's more than just being able to pass and manipulate functions. I need to think like a tree.