http://plato.stanford.edu/entries/set-theory/
http://www.rapidtables.com/math/symbols/Set_Symbols.htm
https://en.wikipedia.org/wiki/Set_theory
http://www.math.toronto.edu/weiss/set_theory.pdf
set theory
原文：http://www.cnblogs.com/WCFGROUP/p/5469487.html
