def get_last_usable_skill(skill_dict):
    """Returns the last usable skill contained by the input dictionary.

    Args:
        skill_dict: an ordered dictionary with scox.value.Skill objects as
        values.

    Returns: the last usable skill in skill_dict.
    """
    usable = []
    for s in skill_dict.keys():
        if skill_dict[s].is_usable():
            usable.append(s)
    if len(usable) > 0:
        return usable[-1]
    else:
        return None