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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 encipher(self,string,keep_punct=False):
 r"""Encipher string using rot13 cipher. Example:: ciphertext = Rot13().encipher(plaintext) :param string: The string to encipher. :param keep_punct: if true, punctuation and spacing are retained. If false, it is all removed. Default is False. :returns: The enciphered string. """
        
        if not keep_punct: string = self.remove_punctuation(string)
        ret = ''
        for c in string:
            if c.isalpha(): ret += self.i2a( self.a2i(c) + 13 )
            else: ret += c
        return ret