XPath guide and playground

Hi, I was trying to optimize my forms by adding some display conditions. We have some psychologists who are in a case sharing group so they should see the other forms but no one should be enter their forms other than them.

This is a new case list I made specifically for them, so now I need to make a display condition to check the username (there is one mobile worker for the psychologists for now), or better yet check the user properties.

I'm trying this
count(instance('commcaresession')/session/user/data/pss) > 0 and instance('commcaresession')/session/user/data/pss = 'yes'
but it doesn't work even if I login with the correct user with this setting set to 'yes'.
It works with another case list with the exact same line but with a different property, but that also worked after around 10 times of me mindlessly cutting and pasting the same line in the field.

My question beside this issue is how I can find a guide on these expressions and a playground to run tests on. The random scattered lines of code, most of which are probably old and don't work don't really help.