what constitutes integrity?

[Thien-Thi Nguyen]

RCS file grammar (comma-v format, rcsfile(5)) top-level was frozen
w/ RCS 5.8 (released 2011-08-30).  The escape hatch is the phrase:

  integrity {string};

for which the string sub-format is not yet specified, added w/ the
intent of future (backward-compatible) support for one of:

  integrity;
  integrity @CACHED-HASH-INFO@;

where CACHED-HASH-INFO is an incrementally maintained blob that
encodes meta-information pertaining to each revision in the
comma-v file, as well as their relationships (branch points, etc).
This can be used for tamper-resistence/detection, synchronization
(e.g., to support "distributed" usage), and so on.

So, what do people think should go into CACHED-HASH-INFO?  Why?
When should it be consulted/updated?  What happens on discrepency?

Just as important: What should NOT be included (and why not)?

I imagine some designs will depend on whether or not an external
file is admissible.  For now, let's say no, although people should
feel free to mention the idea, w/ the dependency explicitly noted,
anyway.  (Perhaps the design can be refined towards the ideal of a
completely self-contained comma-v, or perhaps we will eventually
realize that such an ideal is not possible.  I don't know.)