```python
from typing import List, Optional

def cell_at(grid: List[List[int]], row: int, column: int) -> Optional[int]:
    if not (0 <= row < len(grid)) or not (0 <= column < len(grid[0])):
        return None
    return grid[row][column]

def diag_of(grid: List[List[int]]) -> List[int]:
    return [grid[i][i] for i in range(len(grid))]
```