function preValidatePurchase(uint256 _amount) view internal {
        require(_amount > 0);
        require(ctuContract.isOfferingStarted());
        require(ctuContract.offeringEnabled());
        require(currentTokenOfferingRaised.add(ctuContract.currentTokenOfferingRaised().add(_amount)) <= ctuContract.currentTotalTokenOffering());
        require(block.timestamp >= ctuContract.startTime() && block.timestamp <= ctuContract.endTime());
    }