Xpath constraint - cant get it working

In a page with Programma objects I want to you a grid selector to select Cursus objects to add to the reference set but only if both share the same reference to an Opl object. I tried many ways but it does not work, reason I Need xpath as source for the grid is the option to count/sum columns... I solved the reference problem for now but I am curious what the xpath shoueld be, if possible... link text
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