<think>
Okay, I need to implement two functions, cell_at and diag_of, for a grid. Let's start with cell_at. The function should return the value at a specific row and column. But first, I need to check if the row and column are within the grid's bounds. If either is out of range, return None. So, for a grid, the rows are from 0 to len(grid)-1, and columns are 0 to len(grid[0])-1. Wait, but what if the grid is empty? Then, if the grid is empty, any row or column would be out of bounds. So, first, check if the grid is empty. If it's empty, return None. Otherwise, check if row is between 0 and len(grid)-1, and column is between 0 and len(grid[row])-1. Wait, because each row might have different lengths? Or is the grid a square? The problem says that diag_of returns the main diagonal, but the grid might not be square. Hmm, the problem says that diag_of returns the values running down the main diagonal from the top left, one for each row the grid holds. So, for a non-square grid, the main diagonal would be the elements where row == column, but only as long as the column index is within the row's length. So, for each row in the grid, the column index must be valid for that row. So, for cell_at, the grid is a list of lists. So, first, check if the grid is empty. If it's empty, return None. Then check if row is within 0 to len(grid)-1. Then, check if column is within 0 to len(grid[row])-1. If either is invalid, return None. Otherwise, return grid[row][column]. 

Now, for diag_of. The main diagonal is from the top-left to the bottom-right. But if the grid is not square, the diagonal would be the elements where row == column, but only as long as the column index is valid for that row. So, for each row in the grid, if the row's length is at least column+1 (since column starts at 0), then take that element. So, for each row in the grid, check if the column (which is the same as the row index) is within the row's length. So, for row in range(len(grid)), check if column (row) is less than len(grid[row]). If yes, add grid[row][row] to the result. Otherwise, skip. So, the diagonal is the list of grid[i][i] for i in 0 to len(grid)-1, but only if grid[i] has at least i+1 elements. Wait, but if the grid is not square, some rows might have fewer elements. For example, if the grid is 3 rows, with the first row having 2 elements, the second row having 3 elements, and the third row having 2 elements. Then, the main diagonal would be [grid[0][0], grid[1][1], grid[2][2]] but only if each of those columns exist. So, for row 0, column 0 is valid. For row 1, column 1 is valid. For row 2, column 2 is not valid (since the third row has only 2 elements, indexes 0 and 1). So, the diagonal would be [grid[0][0], grid[1][1]].

So, for diag_of, iterate over each row index i. For each i, check if the row i exists (i < len(grid)), and check if the column i is within the row