5. Inheritance, interfaces, abstract contracts and libraries

interface IPriceFeed {                               // no implementation, all functions external
    function latestPrice() external view returns (uint256);
}

abstract contract Pausable {                         // can't be deployed; may have unimplemented functions
    bool public paused;
    modifier whenNotPaused() { require(!paused, "Paused"); _; }
    function _pause() internal virtual { paused = true; }
}

contract Shop is Pausable, Ownable {                 // multiple inheritance, C3 linearization (right-most is most derived)
    constructor() Ownable(msg.sender) {}
    function pause() external onlyOwner { _pause(); }
    function _pause() internal override { super._pause(); emit ShopPaused(); }   // virtual/override, super
    event ShopPaused();
}

library MathLib {                                    // reusable code; internal functions are inlined
    function percent(uint256 x, uint256 bps) internal pure returns (uint256) { return x * bps / 10_000; }
}
using MathLib for uint256;                           // attach: amount.percent(250)