src/vs/editor/contrib/snippet/browser/snippetParser.ts/TextmateSnippet/enclosingPlaceholders
class TextmateSnippet:	enclosingPlaceholders(placeholder: Placeholder): Placeholder[] {
		const ret: Placeholder[] = [];
		let { parent } = placeholder;
		while (parent) {
			if (parent instanceof Placeholder) {
				ret.push(parent);
			}
			parent = parent.parent;
		}
		return ret;
	}