function refund() external {
        require((contribution[msg.sender] > 0) && (!allSaleCompleted) && (block.timestamp > end_time)  && (totalTokens < tokenGenerationMin));
        uint256 tokenBalance = balances[msg.sender];
        uint256 refundBalance = contribution[msg.sender];
        balances[msg.sender] = 0;
        contribution[msg.sender] = 0;
        totalTokens = safeSub(totalTokens, tokenBalance);
        WolkDestroyed(msg.sender, tokenBalance);
        LogRefund(msg.sender, refundBalance);
        msg.sender.transfer(refundBalance); 
    }