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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 related_units(relid=None):
 """A list of related units"""

    relid = relid or relation_id()
    units_cmd_line = ['relation-list', '--format=json']
    if relid is not None:
        units_cmd_line.extend(('-r', relid))
    return json.loads(
        subprocess.check_output(units_cmd_line).decode('UTF-8')) or []