let doc = new Tree([
  new Rule('.example', [
    new Property('width', new Value(1, 'px')),
    new Property('height', new Value(2, 'px')),
  ])
]);

// Select the "width" property out of the rule.
let child = doc.children[0].children[0];

// From "child", how do we get back to the rule?
// This normally would be impossible in JavaScript.
// But with the `Tree` initialization, we can use
// .parent to get a reference back to the parent.
let parent = child.parent;

console.log(parent.name);
// -> ".example"