Let s be a string or other arraylike object, a, b > 0 integers, s[0:b] returns a blong prefix to s, the same as s[:b], but s[a:0] returns empty (for len(s) > 0 and a > 1), while it should return the same as s[a:], an along suffix (a > 0).
A syntax asymmetry like this shall not be imposed to those using nonliteral slicing indexes, as it would be necessarily to introduce a control condition to test whether the upper bound index is a nonnegative quantity, assuming the lower bound index is negative. Furthermore, it breaks the whole negative slicing idea, being that (I consider) index i always be treated as i mod len(s), so that constructions like s[a:b] (for a, b > 0 or a, b < 0) could return s[a:] + s[:b].
