Row security and hierarchies-part 1

Fair warning: this is going to be complicated. In this post, I am going to assume that you've read the row security whitepaper, you are familiar with how parent/child hierarchies work in the tabular model, and you can deal with fairly complicated DAX concepts. For a primer on hierarchies, see Kasper's post. For DAX, I recommend a few months of experience after learning DAX. If you don't have that much DAX experience, let your eyes gently glaze over while I try to explain the DAX formulas, blindly copy/paste the formulas into your model, and hope for the best without trying to achieve true understanding (the latter is generally how I deal with DAX). Now to the post.