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Go ahead and check it in.

Strings and unicode also need to support slices. While
"hello"[::2] is silly, it's useful to be able to write
"hello"[slice(1,-1)].

I think array needs more work -- first 'array.array' should
be made into a type object that can be invoked as a
constructor and subclassed. That would be nice, yes, but is
a lower priority for me ATM.

Off-topic aside: the change you had to make to
PyString_Format() and its Unicode cousin suggests that we
need a better way to decide if something is really a
mapping...  E.g. this now gives a confusing error message:
  "%(foo)s %(bar)s" % [1,2,3]
I wonder how many other places in the code make naive
mapping tests (and of course operator.isMappingType becomes
totally unreliable). Maybe the presence of a keys() method
together with a __getitem__ method should be used as a
standard test for mapping-ness?