<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def short_title(self):
 """ Generates an abbreviated title by subtracting the parent's title from this instance's title. """

        if self.title and self.parent is not None and hasattr(self.parent, 'title') and self.parent.title:
            if self.title.startswith(self.parent.title):
                short = self.title[len(self.parent.title):].strip()
                match = _punctuation_re.match(short)
                if match:
                    short = short[match.end():].strip()
                if short:
                    return short
        return self.title