A service must accept any object that can produce a reference, whether or not it uses For…

A service must accept any object that can produce a reference, whether or not it uses FormatsReference. What should its parameter require?

1. A Referenceable interface implemented by each accepted class
2. The FormatsReference trait name as the parameter type
3. ReflectionClass::getTraits() at every call site
4. A common parent class used only to share the formatter