update TODO list

TODO.md

@@ -15,6 +15,12 @@ A list of things to do for Speculate.
 
 * (interface) print errors on stderr, not on stdout
 
+* (bug) the following now hangs on equivalencesBetween
+
+        make eg/arith && long ./eg/arith -TSs7
+
+  There are `4213394` canonicalVariations of `((_ + _) * _ + (_ + _) * _ :: Int,(_ + _) * _ + (_ + _) * _ :: Int)`.  Testing them all is perhaps not worth it.
+
 * (performance) note that variable assignments form a lattice.  So I only
   need to test stuff from upper if the lower is true.  Of course, testing is
   the expensive thing.  But it does not pay off to test x + y = z + w before