Generate the set of fields that must stay the same across an update.

def namedb_update_must_equal( rec, change_fields ):
    """
    Generate the set of fields that must stay the same across an update.
    """
    
    must_equal = []
    if len(change_fields) != 0:
        given = rec.keys()
        for k in given:
            if k not in change_fields:
                must_equal.append(k)

    return must_equal