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I don't see this as worth cluttering up slice notation.  I like gaul's suggestion 
of using a list comprehension to build your list.  Heck, you can even simplify 
the list comp to ``[a[x] for x in b]``.  Yes, it is not as short as ``a(b)``, but 
writing it out it looks like ``[10, 20, 30, 40, 50, 60, 70, 80, 90][[0, 4, 5, 
7]]`` which just looks *really* cluttered to me.

You also have the built-in 'slice' if you need real control over your slices.  So 
you could do your ``a[0:4, 3:4, 0:-1, -1]`` example as ``[a[slice(*args)] for 
args in [(0,4), (3,4), (0,-1), (-1,)]]``.

God I love list comps.  =)