ERC165.sol

pragma solidity >=0.7.0 <0.9.0;

interface IERC165 {
    function supportsInterface(bytes4 interfaceId) external view returns (bool);
}

interface IERC721 is IERC165 {
    event Transfer(address indexed from, address indexed to, uint256 indexed tokenId);

    event Approval(address indexed owner, address indexed approved, uint256 indexed tokenId);

    event ApprovalForAll(address indexed owner, address indexed operator, bool approved);

    function balanceOf(address owner) external view returns (uint256 balance);

    function ownerOf(uint256 tokenId) external view returns (address owner);
    
    function transferFrom(
        address from,
        address to,
        uint256 tokenId
    ) external;
    
    function approve(address to, uint256 tokenId) external;
    
    function setApprovalForAll(address operator, bool _approved) external;
    
    function getApproved(uint256 tokenId) external view returns (address operator);
    
    function isApprovedForAll(address owner, address operator) external view returns (bool);
}

contract ERC721 {
    IERC721 i;

    function a () view external returns (bytes4){
        return i.balanceOf.selector ^
               i.ownerOf.selector ^
               i.transferFrom.selector ^
               i.approve.selector ^
               i.setApprovalForAll.selector ^
               i.getApproved.selector ^
               i.isApprovedForAll.selector;
        
        // i.safeTransferFrom is overloaded
    }

    function b () pure external returns (bytes4) {
        return type(IERC721).interfaceId;
    }

    function c () pure external returns (bytes4) {
        return this.a.selector;
    }
}