function sell(uint amount) returns (uint revenue){
if (balanceOf[msg.sender] < amount ) throw;        // checks if the sender has enough to sell
balanceOf[this] += amount;                         // adds the amount to owner's balance
balanceOf[msg.sender] -= amount;                   // subtracts the amount from seller's balance
revenue = amount * sellPrice;
if (!msg.sender.send(revenue)) {                   // sends ether to the seller: it's important
    throw;                                         // to do this last to prevent recursion attacks
} else {
    Transfer(msg.sender, this, amount);             // executes an event reflecting on the change
    return revenue;                                 // ends function and returns
}