Black Hat is a fucking joke!

“Exactly,” responded Alexander. “We stand for freedom.”
“Bullshit!” McCoy shouted.
“Not bad,” Alexander said, as applause broke out in the crowd.
[...]
“Read the constitution!” shouted McCoy in one last heckle.
“I have. So should you,” responded Alexander to another round of applause.


http://www.forbes.com/sites/andygreenberg/2013/07/31/nsa-director-heckled-at-conference-as-he-asks-for-security-communitys-understanding/

The first of these results is apparent by considering, for instance, the tangent function, which provides a one-to-one correspondence between the interval (−π/2, π/2) and R (see also Hilbert's paradox of the Grand Hotel).

Symmetric difference of sets A and B, denoted A △ B or A ⊖ B, is the set of all objects that are a member of exactly one of A and B (elements which are in one of the sets, but not in both). For instance, for the sets {1,2,3} and {2,3,4} , the symmetric difference set is {1,4} . It is the set difference of the union and the intersection, (A ∪ B) \ (A ∩ B) or (A \ B) ∪ (B \ A).

Topos also give a natural setting for forcing and discussions of the independence of choice from ZF, as well as providing the framework for pointless topology and Stone spaces.

Not every situation requires the axiom of choice. For finite sets X, the axiom of choice follows from the other axioms of set theory. In that case it is equivalent to saying that if we have several (a finite number of) boxes, each containing at least one item, then we can choose exactly one item from each box.