Clean Separation between URCs and URNs

• URCs should not depend on URNs, even if they were defined.
• URNs should not depend on URCs, even if they were defined.
• URIs are OK - generic identifiers, no specific properties.
• Does resolution of URN always return URC? No!
• Does resolution of URL sometimes returns URC? Yes!
• HTTP extension? - header to indicate "This is metadata".