__Comment A:__

Most students failed to correctly identify invalid inputs. The three test
cases in the autograder that caused the most problems were...

-1 0
0 1
4 a

- If you received a '7', you most likely didn't print out an error message for
any of the three cases.
- A score of '8' was a bit rarer, and usually means that you handled the case
of '-1 0' but missed the other two.
- A score of '9' was usually a failure to handle case '0 1'.

The most common errors for the cases were...
-1 0: No error message. 
0 1:  Printed '1'. 
4 a:  Printed row 0 along with four additional rows of 0's. 

If you run these cases and you find that your reader has made a mistake,
please send him an e-mail.