The builtin `map` function takes one or more user-supplied iterators, and returns an iterator.

When all the user-supplied iterator arguments to `map` provide a valid length hint (according to the PEP 424 length hint protocol), the iterator returned by `map` should provide a length hint.  

I suggest the following behavior:  when all the iterator arguments provide a valid length hint, the map iterator should return the minimum value among all length hint values, because it stops upon exhaustion of the shortest input iterator.

If any user-supplied iterator does *not* provide a length hint according to the PEP 424 protocol, the map iterator should return the `NotImplemented` singleton, in accordance with the protocol.

When the evaluation of `__length_hint__()` for a user-supplied iterator raises an exception, the exception should be propagated by the map iterator's `__length_hint__()` method.