def append_string(self, field, header=False):
        """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."""

        # 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
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.