Add (first order) logic

[Vinno97]

First of all, I think this repository is a great initiative to help programmers delve into and understand theoretical (computer science) literature. However, it would be great if there were more content about the symbols used in (First Order) Logic. Logic sentences are often comprised of just these symbols, making them very hard to read without an intuitive understanding of them.

Personally, I find it very helpful to think about them programmatically when reading them, though not all symbols are equally simple to mentally model.

http://tug.ctan.org/info/undergradmath/undergradmath.pdf provides a nice summary of symbols

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#12 is an example of another person with difficulties in this area.