def is_in(item, a_list):
    """
    Checks whether the item is in the ordered
    list a_list using a binary search.

    Returns Boolean.
    """
    if len(a_list) == 0:
        return False
    elif len(a_list) == 1:
        return item == a_list[0]
    else:
        if item < a_list[len(a_list) / 2]:
            return is_in(item, a_list[: len(a_list) / 2])
        else:
            return is_in(item, a_list[len(a_list) / 2 : ])