@Override
        public R visitRef(Reference reference) {
            return getDefault();
        }
Visits a library function.

@param fn the library function to visit.
@param args the arguments to the function being visited.
@return the value from the visitor.
smithy-rules-engine/src/main/java/software/amazon/smithy/rulesengine/language/syntax/expressions/ExpressionVisitor.java