Upon reading in the input, the trickiest part is determining where exactly the ~T~ and ~B~ strings should be overlaid onto the image grid, such that they end up centered as required.

Let ~|T|~ be the number of characters in ~T~. Then, the first character of ~T~ should replace character ~s = \lfloor \frac{C-|T|}{2} \rfloor+1~ in the second row of the image grid (~1~-indexed). At that point, we can iterate over each character ~T_i~ (for ~i = 1 \dots |T|~), and if ~T_i~ isn't an underscore, we should replace character ~s+i-1~ in the second row of the image grid with ~T_i~.

The same process can be repeated to overlay ~B~ onto the second-last row of the image grid. Following that, we can proceed to output the entire updated image grid, one row at a time.