如何总结XSL中for-each循环的结果？

I am new to XSL so I don´t really know how to do this. I have a for-each statement that does some computations for each element of a type "cell". How can I sum-up the results and store them in a variable so I can display it? I have included a part of the code. I am new to XSL so I don´t really know how to d