private static ExpressionNode getArgExp(Reference reference, int idx) {
        if (reference.arguments().size() > idx) {
            return reference.arguments().expressions().get(idx);
        }
        return null;
    }
Returns the binding if this reference is a simple identifier which is bound in this context.
Returns empty otherwise.
config-model/src/main/java/com/yahoo/schema/MapEvaluationTypeContext.java