set and Sequence theory
https://en.wikipedia.org/wiki/Class_(set_theory)
https://en.wikipedia.org/wiki/Zermelo%E2%80%93Fraenkel_set_theory
https://books.google.it/books?id=dVncCl_EtUkC&pg=PA69
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
https://www.encyclopediaofmath.org/index.php/Axiomatic_set_theory
http://plato.stanford.edu/entries/set-theory/
http://people.umass.edu/gmhwww/595t/pdf/Set-Theory-Chap0.pdf
http://math.stackexchange.com/questions/1353289/set-theory-formula
https://en.wikipedia.org/wiki/Set_theory
