function transferCheck(address _sender, address _receiver, uint256 _value) 
        private
        constant 
        returns (bool safe) 
    {
        require(_value > 0);
        
        require(_receiver != address(0));
        require(sub(balances[_sender], _value) >= 0);
        require(add(balances[_receiver], _value) > balances[_receiver]);
        return true;
    }