docs: def build(self, builder):
          """
          Build XML by appending to builder
          """
          builder.start("Flag", {})
  
          if self.flag_type is not None:
              self.flag_type.build(builder)
  
          if self.flag_value is None:
              raise ValueError('FlagValue is not set.')
          self.flag_value.build(builder)
  
          builder.end("Flag")