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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 _convert_unit(unit):
 """Convert different names into SI units. Parameters unit : str unit to convert to SI Returns ------- str unit in SI format. Notes ----- SI unit such as mV (milliVolt, mVolt), μV (microVolt, muV). """

    if unit is None:
        return ''

    prefix = None
    suffix = None
    if unit[:5].lower() == 'milli':
        prefix = 'm'
        unit = unit[5:]
    elif unit[:5].lower() == 'micro':
        prefix = mu
        unit = unit[5:]
    elif unit[:2].lower() == 'mu':
        prefix = mu
        unit = unit[2:]

    if unit[-4:].lower() == 'volt':
        suffix = 'V'
        unit = unit[:-4]

    if prefix is None and suffix is None:
        unit = unit
    elif prefix is None and suffix is not None:
        unit = unit + suffix
    elif prefix is not None and suffix is None:
        unit = prefix + unit
    else:
        unit = prefix + suffix

    return unit