How to identify which properties are references

From my reading of the JF2 Note at https://www.w3.org/TR/jf2/ I am confused how a parser is supposed to expand references. Are you just supposed to exact-string-match across the values of all properties, then check if that value is a key within `references`?

There are two sub-issues that I can identify here:

- `references` is optional. Does this mean that if it is missing, then you MUST embed the full representation within the property referring to that object? For example, `"author": "http://alice.example.com"` could be a reference, but you have no way of knowing this for sure, unless you specify which properties can be (are always?) references -- things like `author`, `like-of`, and probably others.
- The JSON-LD context document likewise does not define which properties are references, using `@type: @id` within the term definitions. The default value of properties like `author` is assumed to be a string literal `@value`. This leads to a conceptual confusion where you can't tell whether any property specifically refers to a Thing or a String.

I think it would be useful to define which properties are (always?) references.

Start with the JF2 Note and its JSON-LD context, focusing on how references and properties such as author and like-of are defined. Done means the specification clearly states how to identify reference-valued properties and how parsers handle missing references.

Written by the indexing model from the issue text.