TODO item.

TODO

 * ErrorReporting: experiment with merging several symbols
   so as to reduce the noise (e.g. operators, expressions,
-  etc.)
+  etc.). Or just print the same way, but don't quotient
+  internally, as the locations are not the same.
+  Think about HTML output with mouse-over.
+  Also, sort the explanations by a certain measure of
+  complexity. E.g., those that go back to a "higher" non-terminal
+  symbol should be preferred.
 
 * Add left-recursive lists to the standard library.