contract UpgradeabilityProxy is Proxy {
    /**
    * @dev This event will be emitted every time the implementation gets upgraded
    * @param implementation representing the address of the upgraded implementation
    */
    event Upgraded(address indexed implementation);

    // Storage position of the address of the current implementation
    bytes32 private constant IMPLEMENTATION_POSITION = keccak256("mintable.erc721.proxy.implementation");

    /**
    * @dev Constructor function
    */
    constructor() public {}

    /**
    * @dev Tells the address of the current implementation
    * @return address of the current implementation
    */
    function implementation() public view returns (address impl) {
        bytes32 position = IMPLEMENTATION_POSITION;
        assembly {
            impl := sload(position)
        }
    }

    /**
    * @dev Sets the address of the current implementation
    * @param _newImplementation address representing the new implementation to be set
    */
    function _setImplementation(address _newImplementation) internal {
        bytes32 position = IMPLEMENTATION_POSITION;
        assembly {
            sstore(position, _newImplementation)
        }
    }

    /**
    * @dev Upgrades the implementation address
    * @param _newImplementation representing the address of the new implementation to be set
    */
    function _upgradeTo(address _newImplementation) internal {
        address currentImplementation = implementation();
        require(currentImplementation != _newImplementation);
        _setImplementation(_newImplementation);
        emit Upgraded(_newImplementation);
    }
}
/**
 * @title UpgradeabilityProxy
 * @dev This contract represents a proxy where the implementation address to which it will delegate can be upgraded
 */
function _setImplementation(address _newImplementation) internal {
    bytes32 position = IMPLEMENTATION_POSITION;
    assembly {
        sstore(position, _newImplementation)
    }
}

/**
 * @dev Sets the address of the current implementation
 * @param _newImplementation address representing the new implementation to be set
 */