With or without negative indices one can do pointless things with slicing/subseq. Returning an empty string/list is the only sane thing to do in those cases.

    >>> s="aoeui"
    >>> s[3:1]
    ''
    >>> s[4:-3]
    ''
    >>> s[9:-20]
    ''
    >>> s[0:-0]
    ''
    >>> s[12:42]
    ''