docs: 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