Remove old todo items.

TODO

@@ -62,10 +62,6 @@ Division of 0 by 1 reduces to 0.
 
  - Unit tests for strategy.
 
- - MESSAGES needs to be expanded.
-
- - Use pretty-print for expressions in hint messages.
-
  - Parser: add unit tests for operator associativity (derivatives/integrals).
 
  - Modules:
@@ -78,10 +74,6 @@ Division of 0 by 1 reduces to 0.
    or, fix parser error using the following notation:
    int_-1^2 ... dx
 
- - Frontend: implicit rules are applied after expicit ones, but are still
-   printed separately. This yields multiple successive occurences of the same
-   expression.
-
  - Base hint selection on the desired final expression.
    To generate a hint for expression list A, ..., B:
    1. Create a possibility tree for A.