code: def add_raw(self, length_tag, value_tag):
          
  
          self.raw_len_tags.append(length_tag)
          self.raw_data_tags.append(value_tag)
          return
docs: Define the tags used for a private raw data field.
  
          :param length_tag: tag number of length field.
          :param value_tag: tag number of value field.
  
          Data fields are not terminated by the SOH character as is usual for
          FIX, but instead have a second, preceding field that specifies the
          length of the value in bytes.  The parser is initialised with all the
          data fields defined in FIX.5.0, but if your application uses private
          data fields, you can add them here, and the parser will process them
          correctly.