def flatten(iterable):
    """
    Flatten a sequence, returning a sequence of all its items' items.
    
    For example, `flatten([[1, 2], [3], [4, 'meow']]) == [1, 2, 3, 4, 'meow']`.
    """
    iterator = iter(iterable)
    try:
        first_item = next(iterator)
    except StopIteration:
        return []
    return sum(iterator, first_item)
