Hide-when formulas often give me headaches. It sometimes seems so much effort just to decide whether or not I have to show a line of text. And there’s always a condition you forget :-).
Hide-when’s tend to get complex. If they get too long or complex, you might want to reconsider your design. I once saw an application with hide-when formulas that needed 2 pages when printed. Needless to say I threw them out…
What often works for me, is writing a show-when formula: in many occasions it’s easier to know when you want to show something, than when you want to hide it.
You can create a show-when formula by adding !() around your formula, like in:
… meaning that, if the field Placenta has the value Arbitrary the line will be shown.
I suppose this is not a new trick (I did some research before writing this, and I wrote it in at least one other place), but can be handy for some of you.