src/vs/workbench/api/common/extHostNotebookDocument.ts/ExtHostNotebookDocument/version
class ExtHostNotebookDocument:	get apiNotebook(): vscode.NotebookDocument {
		if (!this._notebook) {
			const that = this;
			this._notebook = {
				get uri() { return that.uri; },
				get version() { return that._versionId; },
				get notebookType() { return that._notebookType; },
				get isDirty() { return that._isDirty; },
				get isUntitled() { return that.uri.scheme === Schemas.untitled; },
				get isClosed() { return that._disposed; },
				get metadata() { return that._metadata; },
				get cellCount() { return that._cells.length; },
				cellAt(index) {
					index = that._validateIndex(index);
					return that._cells[index].apiCell;
				},
				getCells(range) {
					const cells = range ? that._getCells(range) : that._cells;
					return cells.map(cell => cell.apiCell);
				},
				save() {
					return that._save();
				}
			};
		}
		return this._notebook;
	}