```\n// Verify \_extraData is a call to unqualifiedDepositToTbtc.\nbytes4 functionSignature;\nassembly { functionSignature := mload(add(\_extraData, 0x20)) }\nrequire(\n    functionSignature == vendingMachine.unqualifiedDepositToTbtc.selector,\n    "Bad \_extraData signature. Call must be to unqualifiedDepositToTbtc."\n);\n```\n