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The docs read better with "vague" words like "end".  Making
the description more mathematical may make it more precise
but will also make it harder to just read and understand. 
The wording suggested above is fathomable only after you
already understand how slicing works.  It is the "vague"
words that help achieve the understanding in the first place.

Also, I do not want to clutter this section with examples
intended to compare and contrast slices versus
slice.indices.  It would be okay to add a brief comparative
example to the docs for slice.indices().  Understanding the
latter is aided by a comparison.  In contrast, you can get
pretty good with slices and know nothing about
slice.indices() which is a somewhat challenging method.