<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def change_meta(self, para, new_value, log=True):
 """ Changes the meta data This function does nothing if None is passed as new_value. To set a certain value to None pass the str 'None' Parameters para: str Meta data entry to change new_value: str New value log: boolean, optional If True (default) records the meta data change in the history """

        if not new_value:
            return
        para = para.lower()
        if para == 'history':
            raise ValueError(
                'History can only be extended - use method "note"')
        old_value = self._content.get(para, None)
        if new_value == old_value:
            return
        self._content[para] = new_value
        if old_value and log:
            self._add_history(entry_type="METADATA_CHANGE",
                              entry='Changed parameter "{para}" '
                                    'from "{old}" to "{new}"'.format(
                                        para=para,
                                        old=old_value,
                                        new=new_value))