def parse_no(txt):
    """parse a decimal unsigned integer from string,
       e.g. "12", number, e.g. 12, None on error"""
    fields = txt.strip().split()
    if len(fields) > 1:
        return None
    try:
        no = int(fields[0])
    except:
        return None
    if no < 0:
        return None
    return no
