A functions Venn diagram

Whilst twitter and the like can be a huge distraction, I persist with it because of the frequency with which it offers me a new idea to work with. Yesterday I had a quick look and saw a post from who Craig Barton who was obviously attending a session at the UK based MEI Conference. It was simply a photograph, which I have included below....

I retweeted and later in the evening it turned up again as posted by Andy Donahue in our IB Maths Studies Teachers facebook group! Anyway, I liked it because it made me think about using venn diagrams as a teaching tool probably for the first time, which of course has double benefits! What else could the three sets be? What else could we isolate with the intersections? I just wanted to share this quickly for now, but will certainly come back to it at a later date.....