need clarity on how to handle extensions when @context is not present

[llorllale]

Section 17:

If an @context is present, any properties not defined in DID Core, MUST be defined in this context, or in a DID Method specific one.

I think we are missing at least an ELSE branch in this decision tree.

[llorllale]

So for example, how can a non-JSONLD-based sidetree implementation support arbitrary verification method types.