I keep forgetting that 'Dao.String.testNull' returns True on null, not False. As a result, the instance for 'Dao.Interpreter.Executable' 'Dao.Interpreter.IfExpr' was backwards, and so was the definition for the logical-NOT "!" operator. These have been fixed by simply adding a "Prelude.not" expression in the correct spots.
Made further improvements to the unit tests and example programs as I test and repair more parts of the "Dao.Interpreter" module.