<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def append_pair(self, tag, value, header=False):
 """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."""


        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