As a result:
* functions such as `nth` should work correctly on iterators;
* `nth` will raise `IndexError` (in a fashion consistent with `get`)
when the index is out of bounds;
* `take`, etc. will raise `ValueError` instead of returning
an ambiguous value if the index is negative;
* `map`, `zip`, `range`, `input`, `filter` work the same way (Py3k one)
on both Python 2 and 3 (see #523 and #331).
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