```python
def cell_at(grid: list, row: int, column: int) -> int:
    if not (0 <= row < len(grid) and 0 <= column < len(grid[0])):
        return None
    return grid[row][column]

def diag_of(grid: list) -> list:
    if len(grid) != len(grid[0]):
        raise ValueError("Grid must be square")
    return [grid[i][i] for i in range(len(grid))]
```