Adding odd numbers doesn't work. ---- user> (+ 2 2) 4 user> (+ 1 3) ClassCastException ---- Cause: Never implemented odd number adding in the Compiler! See the missing branch in FooExpr. Solution: Fully implemented the branch for odd numbers to be just like even numbers. Considered just getting rid of addition altogether but I guess people use it. Patch: add-odd-3.patch