xbase

dirs/d1/files/f1 exists

PUT /dirs/d2/files/f2
   { xbase: /dirs/d1/files/f1/versions/v1 }
   creates a v1 pointing to d1/f1

POST /dirs/d2/files/f2  (creating a new version)
  {}
  creates a v2 w/o a pointer

POST /dirs/d2/files/f2  (creating a new version)
  { xbase: /dirs/f1/files/f1 }
  creates a v3 (like v1) with a pointer to d1/f1

Proposal
- keep xref as it is today (in "meta")
- add "xbase" as a version-level attribute
  - XID that can point to a Resource or Version
  - MUST be a Resource of the same Resource-type
  - xbase can change from version to version  (version-level choice)
  - look at adding a flag for update(), when xbase is set, to prevent accidental overriding
     (e,g, ?yes-do-it, otherwise it's a readonly version)
  - no mechanism to remove an inherited attribute, but they can override its value