src/vs/workbench/api/common/extHostNotebookConcatDocument.ts/ExtHostNotebookConcatDocument/validatePosition
class ExtHostNotebookConcatDocument:	validatePosition(position: vscode.Position): vscode.Position {
		const startIdx = this._cellLines.getIndexOf(position.line);

		const cellPosition = new types.Position(startIdx.remainder, position.character);
		const validCellPosition = this._cells[startIdx.index].document.validatePosition(cellPosition);

		const line = this._cellLines.getPrefixSum(startIdx.index - 1);
		return new types.Position(line + validCellPosition.line, validCellPosition.character);
	}