import itertools


def flatten(iterable):
    """Flatten the input iterable.
    
    >>> list(flatten([[0, 1], [2, 3]]))
    [0, 1, 2, 3]
    >>> list(flatten([[0, 1], [2, 3, 4, 5]]))
    [0, 1, 2, 3, 4, 5]
    """
    return itertools.chain.from_iterable(iterable)