motivation:
simple CLI program acting as universal calculator
able to solve equations like:
- 7*x=5
- 3.14*1231
- log(5)
only one namely is supported and only +,-,*,/, log() operations. Namely cannot be under logarithm, cannot be squared etc.
It would be useful as fast alternative to wolframalpha in math, it it implemented more features like any polynomials.
log() means e-based logarithm


compilation and run:
 g++ main.cpp -std=c++14 && ./a.out


design patterns: 
 - visitor
 - RAII

knowns bugs:
 - implicit multiplication (e.g. 5x) doesn't work

I'm checking against syntax errors, not mathematical errors. Eg. log(-1) is -nan. This behaviour is sometimes desirable. The same with "x=x". If user nan, inf is right float value. This is simple cli app, so user can pipe output to another program, so I prefer to always, when possible return float value, whenever it makes any sense.