function addReferrer(address referrer) external restricted {
        if (players[tx.origin].referrer == address(0) && players[referrer].tickets >= interval && referrer != tx.origin) {
            players[tx.origin].referrer = referrer;
            players[tx.origin].checkpoint = players[tx.origin].tickets;

            emit ReferrerAdded(tx.origin, referrer);
        }
    }