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99/// User pays for their session by providing some data that binds the
100/// payment to the session somehow offchain.
101/// @param paymentConfig_ Must be the same payment config hashed during
102/// construction of the contract.
103/// @param maxTKN_ User can set a maximum TKN that they are willing to
104/// pay for the transaction. As the price is converted by USDT to TKN by
105/// the oracle it is possible for the exchange rate to move against the
106/// user while the transaction is being mined. If the TKN amount exceeds
107/// the limit the transaction will rollback.
108/// @param data_ The session data that binds the payment to the session.
109/// The offchain handling MUST be secure despite the session data being
110/// completely public in the associated `SessionPaid` event. This can be
111/// achieved (for example) by having the `msg.sender` sign a request for
112/// a session key separate to the public session ID in `data_`. This way
113/// the offchain consumer knows that the payer and session key requestor
114/// are the same entity.
115function payTKN(
116    PaymentConfig calldata paymentConfig_,
117    uint256 maxTKN_,
118    bytes calldata data_
119) external onlyValidPaymentConfig(paymentConfig_) {
120    emit SessionPaid(msg.sender, data_);
121
122    // Process payment.
123    uint256 tknAmount_ = calculateUSDTtoTKN(paymentConfig_.usdtPrice);
124    require(tknAmount_ <= maxTKN_, "SessionPaymentPortal: MAX_TKN");
125    SplitPayment.splitTransfer(
126        paymentConfig_.tkn,
127        msg.sender,
128        tknAmount_,
129        paymentConfig_.shares
130    );
131}