Added parser modifications to TODO list.

TODO

@@ -112,4 +112,8 @@ Division of 0 by 1 reduces to 0.
 
  - Use pretty-print for expressions in hint messages.
 
- - Parser: add unit tests for operator associativity (derivatives/integrals).
+ - Parser:
+   + add unit tests for operator associativity (derivatives/integrals).
+   + plus << integral/derivative << times
+   + divide >> times
+   + a2 -> a*2 instead of a^2