Collection<Object> flatArguments() {
        return flatMapToList(args(), a -> {
            if (a instanceof Collection) {
                return (Collection) a;
            }
            return Collections.singleton(a);
        });
    }
Returns a user-facing string representation of this expression, i.e. does not have developer
centric debugging information like the expression id.
indexr-query-opt/src/main/java/io/indexr/query/expr/Expression.java