@staticmethod
def join_uid(prefix, sep, number, digits):
    """Join the parts of an item's UID into a string.
        >>> UID.join_uid('ABC', '', 123, 5)
        'ABC00123'
        >>> UID.join_uid('REQ.H', '-', 42, 4)
        'REQ.H-0042'
        >>> UID.join_uid('ABC', '-', 123, 0)
        'ABC-123'
        """
    return '{}{}{}'.format(prefix, sep, str(number).zfill(digits))
