code: def append_string(self, field, header=False):
          
  
          # Split into tag and value.
          bits = field.split('=', 1)
          if len(bits) != 2:
              raise ValueError("Field missing '=' separator.")
  
          # Check tag is an integer.
          try:
              tag_int = int(bits[0])
          except ValueError:
              raise ValueError("Tag value must be an integer")
  
          # Save.
          self.append_pair(tag_int, bits[1], header=header)
          return
docs: Append a tag=value pair in string format.
  
          :param field: String "tag=value" to be appended to this message.
          :param header: Append to header if True; default to body.
  
          The string is split at the first '=' character, and the resulting
          tag and value strings are appended to the message.