docs: function buildGraph() {
  	const graph = {};
  	// https://jsperf.com/object-keys-vs-for-in-with-closure/3
  	const models = Object.keys(conversions);
  
  	for (let len = models.length, i = 0; i < len; i++) {
  		graph[models[i]] = {
  			// http://jsperf.com/1-vs-infinity
  			// micro-opt, but this is simple.
  			distance: -1,
  			parent: null
  		};
  	}
  
  	return graph;
  }