sequence slicing documentation

[Steven Bethard]

In trying to work out what's different between the start, stop and step 
of slice.indices() and the start, stop and step of sequence slicing[1] I 
found that some of the list slicing documentation[2] is vague.  I'd like 
to submit a documentation fix, but I want to make sure I have it right. 
  Here's what points (3) and (5) of the Sequence Types documentation say 
now:

"""
(3) If i or j is negative, the index is relative to the end of the 
string: len(s) + i or len(s) + j is substituted. But note that -0 is 
still 0.
...
(5) The slice of s from i to j with step k is defined as the sequence of 
items with index x = i + n*k such that $0 \leq n < \frac{j-i}{k}$. In 
other words, the indices are i, i+k, i+2*k, i+3*k and so on, stopping 
when j is reached (but never including j). If i or j  is greater than 
len(s), use len(s). If i or j are omitted then they become ``end'' 
values (which end depends on the sign of k). Note, k cannot be zero.
"""

I'd like to replace that second-to-last sentence in point (5) with the 
vague ``end'' description with something more explicit.  I'd like it to 
read something like:

"""
If k is positive and i or j is omitted, they will be replaced with 0 and 
len(s) respectively. If k is negative and i or j is omitted, they will 
be replaced with -1 and -len(s)-1 respectively.  Note that these 
replacements happen before the rule from point (3) is applied.
"""

I'd also like to put an example with point (5) to show this behavior in 
action. Something like:

"""
So for example::

     >>> seq = 'abcde'
     >>> len(seq)
     5
     >>> 'edc' == seq[:1:-1] == seq[-1:1:-1]
     True
     >>> 'ca' == seq[2::-2] == seq[2:-5-1:-2]
     True

Note however that manually applying the rule from point (3) (adding 
len(s) to any i or j that is negative) works for i, but does not work 
for j::

     >>> seq[5-1:1:-1]
     'edc'
     >>> seq[2:-1:-2]
     ''

This is because Python sees the -1 in the j position and applies the 
rule from point (3) again.
"""

If a few people could check over my logic here and make sure I'm not 
completely misguided, I'll post a documentation fix.

Thanks,

STeVe

[1] http://mail.python.org/pipermail/python-list/2005-August/293963.html
[2] http://docs.python.org/lib/typesseq.html