Set the standard we'll be using (isupport CASEMAPPING).

def set_std(self, std):
        """Set the standard we'll be using (isupport CASEMAPPING)."""
        if not hasattr(self, '_std'):
            IMap.__init__(self)

        # translation based on std
        self._std = std.lower()

        # set casemapping maps
        self._set_transmaps()

        # create translations
        if self._lower_chars:
            self._lower_trans = str.maketrans(self._upper_chars, self._lower_chars)
        if self._upper_chars:
            self._upper_trans = str.maketrans(self._lower_chars, self._upper_chars)