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

def diag_of(grid: list) -> list:
    if not grid or not all(len(row) == len(grid[0]) for row in grid):
        raise ValueError("Grid must be square")
    return [grid[i][i] for i in range(len(grid))]
```