function _transfer(address _from, address _to, uint _value) internal {
        
        require(_to != 0x0);
        
        require(balanceOf[_from] >= _value);
        
        require(balanceOf[_to] + _value > balanceOf[_to]);
        
        uint previousBalances = balanceOf[_from] + balanceOf[_to];
        
        balanceOf[_from] -= _value;
        
        balanceOf[_to] += _value;
        emit Transfer(_from, _to, _value);
        
        assert(balanceOf[_from] + balanceOf[_to] == previousBalances);
    }