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contracts...ExtraStorage.sol

// SPDX-License-Identifier: GPL-3.0
//REF: https://github.com/spo0ds/Journey-to-become-a-Blockchain-Engineer
pragma solidity ^0.8.3;
import "./SimpleStorage.sol";

contract ExtraStorage is SimpleStorage{
    function incrementAge(uint256 _age) public override{
        age=_age+1;
    }
}