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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_session(session=None):
 r"""Add a session to the SessionActivity table. :param session: Flask Session object to add. If None, ``session`` is used. The object is expected to have a dictionary entry named ``"user_id"`` and a field ``sid_s`` """

    user_id, sid_s = session['user_id'], session.sid_s
    with db.session.begin_nested():
        session_activity = SessionActivity(
            user_id=user_id,
            sid_s=sid_s,
            ip=request.remote_addr,
            country=_ip2country(request.remote_addr),
            **_extract_info_from_useragent(
                request.headers.get('User-Agent', '')
            )
        )
        db.session.merge(session_activity)