The first test appends the list '(b a)' to the list '(c a)' and checks to make sure the head of the list is 'c'.

The second message means: Above, I said, when you append a list to another list, you cannot re-use the nodes from the first list. You must, on the other hand, re-use the nodes from the second list.

Have you possibly made a.append(b) put b first instead of a first?

Your code should share as many nodes as possible, but no more than that. When you append a list to another, you cannot re-use the first list's nodes (unless the second list is empty).

Also, all of the nodes in your l1 should be frozen. You should not be able to create a loop by appending one list to another. You'd need to modify frozen info.