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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 xml(self, operator='set', indent = ""):
 """Serialize the metadata field to XML"""

        xml = indent + "<meta id=\"" + self.key + "\""
        if operator != 'set':
            xml += " operator=\"" + operator + "\""
        if not self.value:
            xml += " />"
        else:
            xml += ">" + self.value + "</meta>"
        return xml