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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 write_to(self, stream):
 """Write an XML representation of self, an ``Event`` object, to the given stream. The ``Event`` object will only be written if its data field is defined, otherwise a ``ValueError`` is raised. :param stream: stream to write XML to. """

        if self.data is None:
            raise ValueError("Events must have at least the data field set to be written to XML.")

        event = ET.Element("event")
        if self.stanza is not None:
            event.set("stanza", self.stanza)
        event.set("unbroken", str(int(self.unbroken)))

        # if a time isn't set, let Splunk guess by not creating a <time> element
        if self.time is not None:
            ET.SubElement(event, "time").text = str(self.time)

        # add all other subelements to this Event, represented by (tag, text)
        subelements = [
            ("source", self.source),
            ("sourcetype", self.sourceType),
            ("index", self.index),
            ("host", self.host),
            ("data", self.data)
        ]
        for node, value in subelements:
            if value is not None:
                ET.SubElement(event, node).text = value

        if self.done:
            ET.SubElement(event, "done")

        stream.write(ET.tostring(event))
        stream.flush()