on slices, negative indices, which are the equivalent procedures?

[Hope Rouselle]

Hope Rouselle <hrouselle at jevedi.com> writes:

> Dennis Lee Bieber <wlfraed at ix.netcom.com> writes:
>
>> On Sun, 15 Aug 2021 00:05:29 -0300, Jack Brandom <jbrandom at example.com>
>> declaimed the following:
>>
>>>Dennis Lee Bieber <wlfraed at ix.netcom.com> writes:
>>>
>>
>>>> subscript: '.' '.' '.' | test | [test] ':' [test] [sliceop]
>>>> sliceop: ':' [test]
>>
>>>
>>>This is looking less readable, so, no, I prefer that previous, which was
>>>much clearer about slices.  I can't even quite make the slices out in
>>>these rules.  (But thanks for the opportunity to try.)
>>
>> 	The last part of "subscript" has the main "slice" notation (stripping
>> the quotes)
>>
>> 		[test] : [test]
>>
>> and the "stride" component is [sliceop] being
>>
>> 		: [test]
>
> Let's see.  Let ``-->'' mean ``expands to''.  Then
>
>       subscript 
>   --> [test] ':' [test] [sliceop]
>   --> [test] ':' [test] ':' [test]
>
> Does the brackets mean that its content count be omitted?
  ^^^^                                    ^^^^^

_Do_ the brackets mean that its content _can_ be omitted?  (This is
really too much coffee.  It's crazy where my attention goes.)