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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 istring(self, in_string=''):
 """Return a string that uses this server's IRC casemapping. This string's equality with other strings, ``lower()``, and ``upper()`` takes this server's casemapping into account. This should be used for things such as nicks and channel names, where comparing strings using the correct casemapping can be very important. """

        new_string = IString(in_string)
        new_string.set_std(self.features.get('casemapping'))
        if not self._casemap_set:
            self._imaps.append(new_string)
        return new_string