def parse_two_nos(txt):
    """parse two comma separated decimal unsigned integers from string,
       e.g. "12,34", return tuple of two numbers, e.g. (12, 34),
       None on error"""
    fields = txt.strip().split()
    if len(fields) > 1:
        return None
    fields = fields[0].split(',')
    if len(fields) != 2:
        return None
    try:
        no1 = int(fields[0])
        no2 = int(fields[1])
    except:
        return None
    if no1 < 0 or no2 < 0:
        return None
    return no1, no2
