Disallowing @vocab in JSON-LD context

[dauwhe]

Ivan Herman wrote (in an email thread):

Per the JSON-LD expansion rules if one uses the (perfectly valid!) @vocab rule in the @context, the situation changes radically. Essentially, this means that, with @vocab, all properties get a possibly fake value. In other words, we should disallow the usage of a specific JSON-LD feature in our format. This is doable, but we should be aware of that.