An iterable object is an object capable of returning its members one at a time.
To do so, it must define an __iter__ method that returns an iterator.
The iterator protocol specifies that, in order to be a valid iterator,
an object must define a __next__ and an __iter__ method (because iterators are also iterable).
Defining an __iter__ method that returns anything else than an iterator will raise a TypeError as soon as the iteration
begins.
Note that generators and generator expressions have both __next__ and
__iter__ methods generated automatically.
