Bug fix: I introduced a bug when re-defining the
'Dao.Interpreter.referenceLookup' function (previously the
'qualRefLookup' function), by evaluating to a runtime error when the
lookup failed. What it used to do was return 'Prelude.Nothing' when the
value that was being looked up turned out to be undefined. The
"defined()" built-in Dao language function relied on a Nothing or
non-Nothing value in order to work. I could have asked "defined()" to
catch the error and return "false", and actually, I am still not sure if
that might be the better way to go. But if returning Nothing instead of
throwing an error was working before, I think I will stick with that
behavior for now.