abstract contract Proxy {
    using AddressUtils for address;

    /**
     * @notice delegate all calls to implementation contract
     * @dev reverts if implementation address contains no code, for compatibility with metamorphic contracts
     * @dev memory location in use by assembly may be unsafe in other contexts
     */
    fallback() external payable virtual {
        address implementation = _getImplementation();

        require(
            implementation.isContract(),
            'Proxy: implementation must be contract'
        );

        assembly {
            calldatacopy(0, 0, calldatasize())
            let result := delegatecall(
                gas(),
                implementation,
                0,
                calldatasize(),
                0,
                0
            )
            returndatacopy(0, 0, returndatasize())

            switch result
            case 0 {
                revert(0, returndatasize())
            }
            default {
                return(0, returndatasize())
            }
        }
    }

    /**
     * @notice get logic implementation address
     * @return implementation address
     */
    function _getImplementation() internal virtual returns (address);
}
/**
 * @title Base proxy contract
 */
function _getImplementation() internal virtual returns (address);

/**
 * @notice get logic implementation address
 * @return implementation address
 */