>  
>  On closer inspection the documentation says that the compare function is
>  supposed to return -1, 0 or 1. However this seems to be either
>  unnecessary, inaccurate, or both, since returning zero and one had no
>  effect, and returning 0 and -1 sorts the list perfectly.
>  

It expects you to write a function similar to C(++)'s strcmp().  It return -1
if x is less than y, 0 if they are equal, and 1 if x is greater than y.

return x < y is either 0 or 1 so you never handle the actual truth of this case
which should return -1.

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