private Optional<String> boundIdentifier(Reference reference) {
        if ( ! reference.arguments().isEmpty()) return Optional.empty();
        if ( reference.output() != null) return Optional.empty();
        return Optional.ofNullable(getBinding(reference.name()));
    }
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