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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def iso_datetime(timestamp=None):
 """ Convert UNIX timestamp to ISO datetime string. @param timestamp: UNIX epoch value (default: the current time). @return: Timestamp formatted as "YYYY-mm-dd HH:MM:SS". """

    if timestamp is None:
        timestamp = time.time()
    return datetime.datetime.fromtimestamp(timestamp).isoformat(' ')[:19]