def flatten(iterables, start_with=None):
    """Takes a list of lists ``iterables`` and returns a list containing elements of every list.

    If ``start_with`` is not ``None``, the result will start with ``start_with`` items, exactly as
    if ``start_with`` would be the first item of lists.
    """
    result = []
    if start_with:
        result.extend(start_with)
    for iterable in iterables:
        result.extend(iterable)
    return result