function areTokensBuyable(uint _roundIndex, uint256 _tokens) internal constant returns (bool)
    {
        uint256 current_time = block.timestamp;
        Round storage round = rounds[_roundIndex];

        return (
        _tokens > 0 &&                                              
        round.availableTokens >= _tokens &&                         
        current_time >= round.startTime &&                          
        current_time <= round.endTime                               
        );
    }