- Build expression tree (consisting of expression nodes).
 - Define all rules in src/rules.py.

 - Fix BisonSyntaxError location tracking.

 - Sort polynom on its exponents.


 - No possibilities found for:

   >>> a2b3 + a2b3
   a ^ 2 * b ^ 3 + a ^ 2 * b ^ 3

 - 2 + 3 + 4 rewrites to 5 instead of 5 + 4