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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 _set_contents(self, force=False):
 """ Encodes all child objects into the contents for this object :param force: Ensure all contents are in DER format instead of possibly using cached BER-encoded data """


        if self.children is None:
            self._parse_children()

        contents = BytesIO()
        for child in self:
            contents.write(child.dump(force=force))
        self._contents = contents.getvalue()
        self._header = None
        if self._trailer != b'':
            self._trailer = b''