```python
def cell_at(grid: list[list[int]], row: int,          if not grid or not grid[ or row < 0 or row >= len(grid):
        raise ValueError("Invalid row index")
    return grid[row][]

def diag_of(grid: list[list[int]]):
):
    if not grid or not grid[:
        raise ValueError("Empty grid cannot be empty")
    return [grid[i][```]
``` for]
``` ]
``````