Calling SET with a constant causes a class cast exception

Support Bug
#1

Calling the SET function with a constant throws a ClassCastException.

select (set(\"test\") as ?result) {}

returns

Uncaught error during query evaluation: ClassCastException: com.complexible.stardog.plan.filter.expr.ConstantImpl cannot be cast to com.complexible.stardog.plan.filter.expr.Variable