def get_set_membership(test_set, reference_set, defaults=None):
    """
    See if test_set has any elements that aren't keys of the reference_set
    or optionally defaults.  Takes lists as arguments
    """
    missing_members = []

    if defaults is None:
        defaults = []
    for member in test_set:
        if member not in reference_set and member not in defaults:
            missing_members.append(member)
    return missing_members