Re: Negative nth index

[Stefan Monnier <monnier@iro.umontreal.ca>]

>>>> Should negative index for nth be valid? e.g.
>>>> (nth -1 '(1 2)) returns 1

[ I'd make it return 2 otherwise you can't use it to return the
  last element.  ]

>>> What does this do in other Lisps?
>> In CL, it errors (ditto for nthcdr with negative index).  Same for (nth
>> '(1 2 3) -1) in Clojure or (list-ref '(1 2 3) -1) in Scheme.
> Stefan Monnier, any comments here?

I don't see a strong argument in favor of making it index from the end
rather than signal an error.  What would the implementation look like?
Would it be significantly more efficient than doing it "by hand" e.g.:

    (let ((l (length X)))
      (nth (if (< i 0) (- l i) i) X))


- Stefan