docs: def createNewOoid(timestamp=None, depth=None):
    """Create a new Ooid for a given time, to be stored at a given depth
    timestamp: the year-month-day is encoded in the ooid. If none, use current day
    depth: the expected storage depth is encoded in the ooid. If non, use the defaultDepth
    returns a new opaque id string holding 24 random hex digits and encoded date and depth info
    """
    if not timestamp:
      timestamp = utc_now().date()
    if not depth:
      depth = defaultDepth
    assert depth <= 4 and depth >=1
    uuid = str(uu.uuid4())
    return "%s%d%02d%02d%02d" %(uuid[:-7],depth,timestamp.year%100,timestamp.month,timestamp.day)