src/vs/workbench/api/common/extHostNotebook.ts/ExtHostNotebookController/getNotebookDocument
class ExtHostNotebookController:	getNotebookDocument(uri: URI, relaxed?: true): ExtHostNotebookDocument | undefined {
		const result = this._documents.get(uri);
		if (!result && !relaxed) {
			throw new Error(`NO notebook document for '${uri}'`);
		}
		return result;
	}