src/vs/editor/common/model/editStack.ts/MultiModelEditStackElement/canAppend
class MultiModelEditStackElement:	public canAppend(model: ITextModel): boolean {
		if (!this._isOpen) {
			return false;
		}
		const key = uriGetComparisonKey(model.uri);
		if (this._editStackElementsMap.has(key)) {
			const editStackElement = this._editStackElementsMap.get(key)!;
			return editStackElement.canAppend(model);
		}
		return false;
	}