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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 get_identifiers(sysmeta_pyxb):
 """Get set of identifiers that provide revision context for SciObj. Returns: tuple: PID, SID, OBSOLETES_PID, OBSOLETED_BY_PID """

    pid = d1_common.xml.get_opt_val(sysmeta_pyxb, 'identifier')
    sid = d1_common.xml.get_opt_val(sysmeta_pyxb, 'seriesId')
    obsoletes_pid = d1_common.xml.get_opt_val(sysmeta_pyxb, 'obsoletes')
    obsoleted_by_pid = d1_common.xml.get_opt_val(sysmeta_pyxb, 'obsoletedBy')
    return pid, sid, obsoletes_pid, obsoleted_by_pid