private Optional<ExpressionFunction> functionInvocation(Reference reference) {
        if (reference.output() != null) return Optional.empty();
        ExpressionFunction function = getFunctions().get(reference.name());
        if (function == null) return Optional.empty();
        if (function.arguments().size() != reference.arguments().size()) return Optional.empty();
        return Optional.of(function);
    }
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