code: function findEnd(node) {
  	if (!node) {
  		return undefined;
  	}
  	var {left, right, el} = node;
  	findEnd(left);
  	findEnd(right);
  	node.high = Math.max(getHigh(left), getHigh(right), el.end);
  	return node;
  }
docs: Find the highest end value of each node. Mutates its input.