code: def decipher(self,string,keep_punct=False):
                  
          if not keep_punct: string = self.remove_punctuation(string)    
          ret = ''
          for c in string:
              if c.isalpha(): ret += self.i2a(self.inva*(self.a2i(c) - self.b))
              else: ret += c
          return ret
docs: Decipher string using affine cipher according to initialised key.
  
          Example::
          
              plaintext = Affine(a,b).decipher(ciphertext)     
  
          :param string: The string to decipher.
          :param keep_punct: if true, punctuation and spacing are retained. If false, it is all removed. Default is False. 
          :returns: The deciphered string.