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

 - Fix BisonSyntaxError location tracking.

 - Sort polynom by 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
   -> the problem is that the 'root' of the application is actually a subtree
   of the entire expression. This means that the parent of each possibility
   root (or 'subtree') must me stored to be able to replace the subtree.

 - MESSAGES needs to be expanded.

 - rewrite match_combine_polynomes to an even more generic form:
   match_combine_factors.