negative index for the sequence (correction)

[Hiroshi G. Okuno <Okuno@SUMEX-AIM.ARPA>]

My original message contains a bug.  The sentence

 > Of course, if the non-negative index which corresponds to a negative one
 > exceeds the bound of the sequence, an error is signalled.

should read

   If (abs <index>) is out-of-range, an error is signalled.

Thanks,

- Gitchang -
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