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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 _to_unicode(instr):
    '''
    Converts from current users character encoding to unicode.
    When instr has a value of None, the return value of the function
    will also be None.
    '''
    if instr is None or isinstance(instr, six.text_type):
        return instr
    else:
        return six.text_type(instr, 'utf8')