function buyback(uint amount) public returns (uint) {
    require(balance[msg.sender] >= amount);

    // Adjust balance and total supply
    balance[msg.sender] -= amount;
    _totalSupply -= amount;

    // Send ether back
    // Wwill fail if the contract doesn't have enough funds
    msg.sender.transfer(amount / rate);

    // Generate transfer event
    Transfer(msg.sender, 0x0, amount);
}