Nice kata, although some more instructions would've been handy. Now I have only 1 test case that fails when I try to submit, and this is an extremely simple one:

Removing 'a' from '(a)': 
   l1.remove(first) is empty

WTF does this mean? Of course, removing 'a' from (a) gives an empty list in my code... I have this testcase myself (which passes):

list1 = new ListNode( 'a' );
Test.expect( list1.remove(list1.head()).isEmpty());
Test.expect( list1.remove('a').isEmpty());

Any tips? Or can somebody (the author?) please paste the code for this failing test case?