docs: def build(self, builder):
          """
          Build XML by appending to builder
          """
  
          params = {}
          if self.signature_id is not None:
              # If a Signature element is contained within a Signatures element, the ID attribute is required.
              params['ID'] = self.signature_id
  
          builder.start("Signature", params)
  
          if self.user_ref is None:
              raise ValueError("User Reference not set.")
          self.user_ref.build(builder)
  
          if self.location_ref is None:
              raise ValueError("Location Reference not set.")
          self.location_ref.build(builder)
  
          if self.signature_ref is None:
              raise ValueError("Signature Reference not set.")
          self.signature_ref.build(builder)
  
          if self.date_time_stamp is None:
              raise ValueError("DateTime not set.")
          self.date_time_stamp.build(builder)
  
          builder.end("Signature")