src/vs/editor/contrib/documentSymbols/outlineModel.ts/OutlineElement/adopt
class OutlineElement:	adopt(parent: TreeElement): OutlineElement {
		let res = new OutlineElement(this.id, parent, this.symbol);
		for (const [key, value] of this.children) {
			res.children.set(key, value.adopt(res));
		}
		return res;
	}