def METHOD_NAME(request: HttpRequest, vcode: str) -> Optional[str]:
    """
    Verify that the given verification code is identical to the one
    stored in the request session. If anything is amiss, return a
    string describing the error; otherwise, return None.
    """
    # Remember that if the user submitted an invalid phone number in the
    # previous step, we don't want to leak that information here.
    req_vcode = request.session.get(VCODE_SESSION_KEY)
    if not constant_time_compare(req_vcode, vcode):
        if req_vcode is not None:
            req_user_id = request.session.get(USER_ID_SESSION_KEY)
            logger.info(f"Invalid verification code for user id {req_user_id}.")
        return "Incorrect verification code!"
    req_ts = request.session.get(TIMESTAMP_SESSION_KEY, 0)
    now = time.time()
    time_elapsed = now - req_ts
    if time_elapsed > VERIFICATION_MAX_SECS:
        return "Verification code expired. Please go back and re-enter your phone number."
    del request.session[VCODE_SESSION_KEY]
    del request.session[TIMESTAMP_SESSION_KEY]
    request.session[VERIFIED_TIMESTAMP_SESSION_KEY] = now
    return None