function sell(uint amount) returns (uint revenue){
            if (balanceOf[msg.sender] < amount ) throw;       
            balanceOf[this] += amount;                        
            balanceOf[msg.sender] -= amount;                  
            revenue = amount * sellPrice;                     
            msg.sender.send(revenue);                         
            Transfer(msg.sender, this, amount);                
            return revenue;                                    
        }