So I have 3 vignettes - Story A, B, and C, each with 3 possible arrangements - 1,2,3. So I want to first display a random Story from any one of the arrangements, say A2. Then I need to display arrangement 1 or 3, but not A1 or A3. Let's say it displays C1, then the final story to be displayed is of course B3. I don't need a situation where the same story is used for two different arrangements Like A1, C2,A3. How do I work out this two dimmensions of randomisation?  I have already used a randomiser to show one story from arrangements 1,2, and 3 each, but I’m not able to ensure that the stories are randomised and not repeated across these arrangements.