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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 clean_up_inverse(self, current):
 """ Clean up current. Python doesn't have variable lookbehinds, so we have to do negative lookaheads. So we use the pattern `(?:(?!(?:stuff|to|exclude)<x>))[^/]*?)` where <x> is everything that comes after the negative group. `!(this|that)other` --> `(?:(?!(?:this|that)other))[^/]*?)`. Lastly we make sure all is accounted for when finishing the pattern at the end. If there is nothing to store, we store `$`: `(?:(?!(?:this|that)$))[^/]*?)`. """


        if not self.inv_ext:
            return

        index = len(current) - 1
        while index >= 0:
            if isinstance(current[index], InvPlaceholder):
                content = current[index + 1:]
                content.append(_EOP if not self.pathname else self.path_eop)
                current[index] = (''.join(content)) + (_EXCLA_GROUP_CLOSE % str(current[index]))
            index -= 1
        self.inv_ext = 0