Thanks, I got there... I'm still not sure how I managed to pass all the other tests trying to populate it from inside out and being off, but when I flipped it over and populated it from outside in everything went smoothly.

Is there something wrong with the dim( 2, 3, 'a'); test cases?

I only fail on the last modify cells test cases... I get 'modified sub-array should be : 0,x,x,x,x,x'

Should the 'x's be 'a's?

I don't see how if [["a","a","a"],["a","a","a"]] is correct for the first test that 0,x,x,x,x,x would be correct for the last test.