src/vs/workbench/api/common/extHostNotebookConcatDocument.ts/ExtHostNotebookConcatDocument/positionAt
class ExtHostNotebookConcatDocument:	positionAt(locationOrOffset: vscode.Location | number): vscode.Position {
		if (typeof locationOrOffset === 'number') {
			const idx = this._cellLengths.getIndexOf(locationOrOffset);
			const lineCount = this._cellLines.getPrefixSum(idx.index - 1);
			return this._cells[idx.index].document.positionAt(idx.remainder).translate(lineCount);
		}

		const idx = this._cellUris.get(locationOrOffset.uri);
		if (idx !== undefined) {
			const line = this._cellLines.getPrefixSum(idx - 1);
			return new types.Position(line + locationOrOffset.range.start.line, locationOrOffset.range.start.character);
		}
		// do better?
		// return undefined;
		return new types.Position(0, 0);
	}