Move the mouse around the page, and watch as OM reactively sets the co-ordinates. This was slightly modified from David Nolan's examples at https://github.com/swannodette/om/blob/master/examples/mouse/src/core.cljs to add a
<div id="app"></div> programmatically. I imagine there's an official OM way to splice components into a page, but used Dommy instead ... FTW.
Compare and contrast to the equivalent Big-Bang version: http://programming-enchiladas.destructuring-bind.org/rm-hull/8617788