mbda x: 1
    >>> reducefunc = sum
    >>> result = map_reduce('abbccc', keyfunc, valuefunc, reducefunc)
    >>> sorted(result.items())
    [('A', 1), ('B', 2), ('C', 3)]

You may want to filter the input iterable before applying the map/reduce
procedure:

    >>> all_items = range(30)
    >>> items = [x for x in all_items if 10 <= x <= 20]  # Filter
    >>> keyfunc = lambda x: x % 2  # Evens map to 0; odds to 1
    >>> categories = map_reduce(items, keyfunc=keyfunc)
    >>> sorted(categories.items())
    [(0, [10, 12, 14, 16, 18, 20]), (1, [11, 13, 15, 17, 19])]
    >>> summaries = map_reduce(items, keyfunc=keyfunc, reducefunc=sum)
    >>> sorted(summaries.items())
    [(0, 90), (1, 75)]

Note that all items in the iterable are gathered into a list before the
summarization step, which may require significant storage.

The returned object is a :obj:`collections.defaultdict` with the
``default_factory`` set to ``None``, such that it behaves like a normal
dictionary.

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