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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 render(self, doc):
 """Render all elements using specified document. @param doc: the writable document to render to. @type doc: document.IWritableDocument @return: a deferred fired with the specified document when the rendering is done. @rtype: defer.Deferred """

        d = defer.succeed(doc)
        for element in self._elements:
            d.addCallback(element.render)
        return d