Here's a proof-based joke my political game theory professor told us.

This proof follows the form of "universal generalization." The proof looks like:
1) Let X be a set, and x an arbitrary element of X.
2) If x has some property, then all elements of X must also have that property. This is trivial, because x was arbitrary.

So, let's apply this proof. Let X be the set of governments, and x an arbitrary government. Since x was arbitrary, clearly x is unjust. Therefore all governments are unjust.