function approve(address _spender, uint256 _value) returns (bool success) {
        if ((_value != 0) && (allowed[msg.sender][_spender] != 0)) 
        {
            return false;
        }
        
        allowed[msg.sender][_spender] = _value;
        Approval(msg.sender, _spender, _value);
        return true;
    }