```ts
export function cellAt(grid: number[][], row: number, column: number): number {
    if (row < 0 || row >= grid.length || column < 0 || column >= grid[row].length) {
        throw new Error("Invalid position");
    }
    return grid[row][column];
}

export function diagOf(grid: number[][]): number[] {
    const result: number[] = [];
    for (let i = 0; i < grid.length; i++) {
        result.push(grid[i][i]);
    }
    return result;
}
```