The reason I am an authority is because I always walk with him from the train station to work.

A fact is that we both have long hair.

Hence a colleague of mine assumed that Long hair is a requirement for people who work here and go by train.

# Venn Diagrams

So we have the following sets:- people who go by train
- people who have long hair
- people who work at my company

# Problems

As I pointed out, the people my colleague, Bart, knows who work at my company and travel by train, also happen to have long hair. But the people my colleague knows is just a subset of people at my company. So let's add the set "people who Bart knows". So we have the following sets:- people who go by train
- people who have long hair
- people who work at my company
**people who Bart knows**

Now, we run into a little problem with Venn diagrams. As always, Wikipedia

^{[1]}to the rescue.

It's not possible to show all possibilities using regular circles, when the sets increase to more than three. In our case, for four possibilities, we can use ellipses.

# Euler Diagrams^{[2]}

Venn Diagrams are a Subset of Euler Diagrams, because Euler Diagrams are a bit more flexible when it comes to showing only some instead of all possible intersections.The relationship between Venn and Euler diagrams can be displayed as an Euler Diagram.

I'm going to stop now, before I break my brain on the recursion.

# References

- [1] Wikipedia - Venn Diagram
- http://en.wikipedia.org/wiki/Venn_diagram
- [2] Wikipedia - Euler Diagram
- http://en.wikipedia.org/wiki/Euler_diagram