code: def append_pair(self, tag, value, header=False):
          
  
          if tag is None or value is None:
              return
  
          if int(tag) == 8:
              self.begin_string = fix_val(value)
  
          if int(tag) == 35:
              self.message_type = fix_val(value)
  
          if header:
              self.pairs.insert(self.header_index,
                                (fix_tag(tag),
                                 fix_val(value)))
              self.header_index += 1
          else:
              self.pairs.append((fix_tag(tag), fix_val(value)))
          return
docs: Append a tag=value pair to this message.
  
          :param tag: Integer or string FIX tag number.
          :param value: FIX tag value.
          :param header: Append to header if True; default to body.
  
          Both parameters are explicitly converted to strings before
          storage, so it's ok to pass integers if that's easier for
          your program logic.
  
          Note: a Python 'None' value will be silently ignored, and
          no field is appended.