5.3.3 Slicings   Download entire grammar as text.  There is
ambiguity in the formal syntax here: anything that looks like an
expression list also looks like a slice list, so any
subscription can be interpreted as a slicing. Rather than
further complicating the syntax, this is disambiguated by
defining that in this case the interpretation as a subscription
takes priority over the interpretation as a slicing (this is the
case if the slice list contains no proper slice nor ellipses).
Similarly, when the slice list has exactly one short slice and
no trailing comma, the interpretation as a simple slicing takes
priority over that as an extended slicing.  The semantics for a
simple slicing are as follows. The primary must evaluate to a
sequence object. The lower and upper bound expressions, if
present, must evaluate to plain integers; defaults are zero and
the sys.maxint, respectively. If either bound is negative, the
sequence's length is added to it. The slicing now selects all
items with index k such that i <= k <