<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def decipher(self,string):
 """Decipher string using Polybius square cipher according to initialised key. Example:: plaintext = Polybius('APCZWRLFBDKOTYUQGENHXMIVS',5,'MKSBU').decipher(ciphertext) :param string: The string to decipher. :returns: The deciphered string. The plaintext will be half the length of the ciphertext. """
         
        string = self.remove_punctuation(string)#,filter='[^'+self.chars+']')
        ret = ''
        for i in range(0,len(string),2):
            ret += self.decipher_pair(string[i:i+2])
        return ret