Sets a code to be filtered on for the conversion.  Note that
        this also sets the 'code_last' attribute and configures to
        ignore the remaining characters of the code.

        :param idx: The index at which the code _begins_.

        :returns: True if the code is valid, False otherwise.

def set_code(self, idx):
        """
        Sets a code to be filtered on for the conversion.  Note that
        this also sets the 'code_last' attribute and configures to
        ignore the remaining characters of the code.

        :param idx: The index at which the code _begins_.

        :returns: True if the code is valid, False otherwise.
        """

        code = self.format[idx:idx + 3]

        if len(code) < 3 or not code.isdigit():
            return False

        self.codes.append(int(code))
        self.ignore = 2
        self.code_last = True
        return True